Digital circuit having correcting circuit and electronic apparatus thereof

ABSTRACT

Provided is a digital circuit ( 30 ) that comprises: a switching circuit ( 31 ) having first transistors ( 32, 33 ) supplied with power supply potentials (VDD, VSS): correcting circuits ( 34, 36 ) connected between an input terminal (IN) inputted with an input signal and control terminals (gates) of the first transistors: capacitors (C 2 , C 3 ) connected between the control terminals and the input terminal: diode-connected second transistors ( 35, 37 ) that are provided between nodes (N 5 , N 6 ) between the capacitors and the control terminals and the power supply potentials and have the substantially same threshold voltage as the first transistors; and switches (SW 2 , SW 3 ) connected in series with the second transistors.

TECHNICAL FIELD

The present invention relates to a digital circuit with a transistor. Inparticular, the invention relates to a digital circuit provided with acorrecting circuit that, in the case of an amplitude of an input signalbeing smaller than that of a power supply voltage and in the case of apower supply voltage being not sufficiently larger than a thresholdvoltage of a transistor that is used, corrects a DC level of an inputsignal to realize a preferable circuit operation.

BACKGROUND ART

So far, inverter circuits that use transistors such as bipolartransistors and field effect transistors (FETs) have been widely used.In FIG. 36 a, a typical example of an existing CMOS inverter circuitthat uses MOSFETs as transistors is shown. A CMOS inverter circuit 200has a PMOSFET 201 having a threshold voltage V_(THP) and an NMOSFET 202having a threshold voltage V_(THN) with these FETs serially connectedbetween a high level power supply potential VDD and a low level powersupply potential VSS (normally V_(THP) is negative and V_(THN) ispositive). A source of the PMOSFET 201 is connected to the high levelpower supply potential VDD and a source of the NMOSFET 202 is connectedto the low level power supply potential VSS. Drains of both of theMOSFETs 201 and 202 are connected to each other and a connection point N(node) thereof is connected to an output terminal OUT. Furthermore, bothgates of the MOSFETs 201 and 202 are connected to an input terminal INto which an input signal that oscillates between a high level inputpotential V_(INH) and a low level input potential V_(INL)is inputted. Inthe present specification, unless stated, “connection” of a circuitelement means “electrical connection”.

An ordinary operation of the CMOS inverter circuit 200 having such aconfiguration is shown in FIG. 36 b and FIG. 36 c. In FIGS. 36 b and 36c, in order to show an ON/OFF state of the MOSFETs 201 and 202, theMOSFETs 201 and 202 each is shown with a sign of a switch. As shown inFIG. 36 b, when to the input terminal IN, a high level input potentialV_(INH) equal to or higher than a value obtained by subtracting anabsolute value of the threshold voltage of the PMOSFET|V_(THP)| from thehigh level power supply potential VDD is inputted, the PMOSFET 201 isturned off and the NMOSFET 202 is turned on to supply a potentialsubstantially equal to the low level power supply potential VSS to theoutput terminal OUT as an output signal. Furthermore, as shown in FIG.36 c, when to the input terminal IN, a low level input potential V_(INL)equal to or lower than a value obtained by adding an absolute value ofthe threshold voltage of the NMOSFET|V_(THN)| to the low level powersupply potential VSS is inputted, the PMOSFET 201 is turned on and theNMOSFET 202 is turned off to supply a potential substantially equal tothe high level power supply potential VDD to the output terminal OUT asan output signal.

However, in the case of an input signal being supplied from, forinstance, an IC and so on of which operating voltage is low, problemsbelow are caused. As shown in FIG. 37 a, in the case of a high levelinput potential V_(INH) inputted to the input terminal IN being smallerthan a value obtained by subtracting an absolute value of the thresholdvoltage of the PMOSFET 201 |V_(THP)| from the high level power supplypotential VDD, in the PMOSFET 201, a gate-source voltage V_(GS) (=gatepotential V_(G)−source potential V_(S))<−|V_(THP)| is realized, thePMOSFET 201 is not turned off. As a result, both the MOSFETs 201 and 202are turned on, and a potential divided by on-state resistances of thePMOSFET 201 and the NMOSFET 202 is outputted to the output terminal OUT,that is, the low level power supply potential VSS is not outputted.Similarly, as shown in FIG. 37 b, in the case of a low level inputpotential V_(INL) inputted to the input terminal IN being higher than avalue obtained by adding an absolute value of the threshold voltage ofthe NMOSFET 202 |V_(THN)| to the low level power supply potential VSS,the NMOSFET 202 is not turned off, both the MOSFETs 201 and 202 areturned on, and the high level power supply potential VDD is notoutputted to the output terminal OUT. Thus, in the case of, because oflevels between input potentials V_(INH), V_(INL) and power supplypotentials VDD, VSS being different, the MOSFETs 201 and 202 of theinverter circuit 200 being not assuredly turned on or off and an outputnot taking a desired value, there are problems in that a circuit in alater stage of the inverter 200 cannot be driven, or an operation ofsuch circuit becomes uncertain. Furthermore, since both the MOSFETs 201and 202 are simultaneously turned on to flow a short current, there iscaused a problem also in that power consumption increases.

In order to overcome the problems as mentioned above, it is proposedthat, in a level shifter circuit that has a first input inverter and asecond output inverter, a DC level of a signal that is inputted from thefirst inverter to the second inverter is converted by use of a capacitor(condenser) and a biasing means (see Japanese Patent ApplicationLaid-Open No. H9-172367). However, in this circuit, since a DC levelconverting capacitor that is connected between a gate of each of thetransistors constituting the second inverter and an output of the firstinverter is always connected to a high level power supply potential or alow level power supply potential with the biasing means, there areproblems in that charge and discharge of the capacitors may adverselyaffect on the dynamic characteristics of the circuit (that is, lower acircuit operation speed), or power consumption due to the charge anddischarge of the capacitors may become a magnitude that cannot beignored. Furthermore, in the case of there being variations in thethreshold voltages of the transistors, the electrostatic capacity ofeach of the capacitors can be conformed with difficulty to acorresponding transistor. Accordingly, there may occur a problem in thata voltage across the DC level converting capacitor cannot be matched toa threshold voltage of the corresponding transistor, and the transistorscannot be accurately turned on or off.

Furthermore, in the inverter circuit 200 shown in FIG. 36 a, in the caseof a power supply voltage (VDD−VSS) being small, for instance, tosuppress the power consumption, and, being not sufficiently large withrespect to the absolute values of the threshold voltages of the MOSFETs201 and 202, even when an amplitude of an input signal inputted to theinput terminal IN is equal to that of a power supply voltage, there mayoccur a problem in that a sufficient current cannot be flowed to theMOSFETs 201 and 202 to drive with high speed. This is due to that it isnot a gate-source voltage V_(GS) that contributes to a current thatflows in the MOSFET but V_(GS)−V_(TH). For instance, in the invertercircuit 200 shown in FIG. 36 a, VDD=3.3 V, VSS=0 V (ground), a thresholdvoltage of the PMOSFET 201 V_(THP)=−2 V, a threshold voltage of theNMOSFET 202 V_(THN)=3 V, a high level input potential V_(INH)=3.3 V, anda low level input potential V_(INL)=VSS=0 V are assumed. In the case ofthe low level input potential V_(INL) being added to the input terminalIN, in the PMOSFET 201, V_(GS)−V_(THP)=3.3−(−2)=−1.3 V is obtained, andthe PMOSFET 201 is thus turned on, whereas in the NMOSFET 202,V_(GS)−V_(THN)=0−3=−3 V is obtained, and the NMOSFET 202 is thus turnedoff. In this case, since the absolute value of the threshold voltage (−2V) of the PMOSFET is sufficiently small with respect to a power supplyvoltage (that is, an amplitude of an input signal), the absolute valueof (V_(GS)−V_(THP)) can become large (1.3 V), accordingly, there iscaused no problem. On the other hand, in the case of a high level inputpotential V_(INH) being added to the input terminal IN, in the PMOSFET201, V_(GS)−V_(THP)=0−(−2)=2 V is obtained, and the PMOSFET 201 is thusturned off, whereas in the NMOSFET 202, V_(GS)−V_(THN)=3.3 −3=0.3 V isobtained, and the NMOSFET 202 is thus turned on. However, sinceV_(GS)−V_(THN) is such small as 0.3 V, a small current flows and theNMOSFET 202 cannot be operated (on) at high speed. It is a matter ofcourse that when amplitudes of the power supply voltage and the inputsignal are made larger, the high-speed operation can be realized, butthe power consumption increases.

DISCLOSURE OF THE INVENTION

The present invention is carried out to overcome problems of the priorart as mentioned above. A primary object of the invention is to providea digital circuit having a switching circuit that uses a transistor,wherein in accordance with relationship between a power supply voltage,an amplitude of an input signal and a threshold voltage of a transistor,the input signal is properly corrected and thereby a proper circuitoperation is realized.

A second object of the invention is to provide a digital circuit havinga switching circuit that uses a transistor, wherein even in the case ofan amplitude of an input signal being smaller than a power supplyvoltage (difference between a high level power supply potential and alow level power supply potential), the transistor can be assuredlyturned on and off.

A third object of the invention is to provide a digital circuit having aswitching circuit that uses a transistor, wherein even in the case of anamplitude of an input signal being smaller than a power supply voltage,the transistor can be assuredly turned on and off without deterioratingthe dynamic characteristics.

A fourth object of the invention is to provide a digital circuit havinga switching circuit that uses a transistor, wherein even in the case ofan amplitude of an input signal being smaller than a power supplyvoltage, a DC level converting capacitor connected to a control terminalof a transistor contained in the switching circuit is charged to aproper value according to a threshold voltage of a correspondingtransistor, and thereby the transistor can be assuredly operated.

A fifth object of the invention is to provide a digital circuit having aswitching circuit that uses a transistor, wherein even in the case of apower supply voltage being not sufficiently large with respect to theabsolute value of a threshold voltage of the transistor, a sufficientcurrent can flow to the transistor to operate the digital circuit withhigh-speed.

In order to achieve the above objects, according to the invention,provided is a digital circuit having a switching circuit connectedbetween an input terminal and an output terminal. The switching circuitincludes a first transistor that is provided with a first terminal, asecond terminal and a control terminal and can be ON/OFF controlled byvarying a potential of the control terminal with respect to the firstterminal. A first power supply potential is inputted to the firstterminal of the first transistor in normal operation, and the signal atthe output terminal depends on whether the first transistor is ON orOFF. In a normal operation, an input signal that oscillates between afirst input potential that turns off the first transistor and a secondinput potential that turns on the first transistor is input to an inputterminal. The digital circuit has a correcting circuit connected betweenthe input terminal and the control terminal of the first transistor. Thecorrecting circuit has a) a capacitor, one terminal of which isconnected to the input terminal and the other terminal of which isconnected to the control terminal of the first transistor and b) atleast one switch for determining a conduction path for setting, in asetting operation prior to a normal operation, electric charges that areaccumulated in the capacitor so that a voltage across thereof may be apredetermined value. In a normal operation, a state of the at least oneswitch is set so as to hold a voltage at across the capacitor.

According to such a configuration, in a setting operation prior to anormal operation, when a voltage across the capacitor is properly set inaccordance with a power supply voltage, an amplitude of an input signal,a threshold voltage of the first transistor and so on, a DC level of theinput signal can be corrected in the normal operation and thereby apreferable circuit operation can be realized. In the normal operation,since a switch is set so as to hold a voltage (or electric charges)between both ends of the set capacitor, there is no concern of thecapacitor adversely affecting on the dynamic characteristics of thedigital circuit (that is, operation speed is not lowered). On thecontrary, the capacitor, being connected in series with parasiticcapacitance of the transistor to lower total capacitance, can contributeto improve the dynamic characteristics. Furthermore, since there is noneed of frequently carrying out the setting operation, power consumptiondue to the setting operation is only slight.

Preferably, the correcting circuit further includes a second transistorthat is provided with a first terminal, a second terminal and a controlterminal, capable of being ON/OFF controlled by varying a potential ofthe control terminal with respect to the first terminal, and has thesame conductivity type and the substantially same threshold voltage asthe first transistor, and the first terminal of the second transistor isconnected to a first power supply potential, and the second terminal andthe control terminal of the second transistor are connected to eachother and connected to a node between the capacitor and the controlterminal of the first transistor. At least one switch includes a firstswitch connected in series with the second transistor, and in a normaloperation, the first switch is turned off.

Typically, the first and the second transistors are constituted of FETs,and each of the first terminals, the second terminals and the controlterminals of the first and the second transistors is constituted of asource, a drain and a gate, respectively. As the power supply potential,a high level power supply potential and a low level power supplypotential are supplied. When an input signal oscillates between a highlevel input potential and a low level input potential, in the case ofthe first transistor being, for instance, a PMOSFET, the first powersupply potential can be set at the high level power supply potential anda first input potential can be set at the high level input potential.Furthermore, in the case of the first transistor being, for instance, anNMOSFET, the first power supply potential can be set at the low levelpower supply potential and the first input potential can be set at thelow level input potential.

According to one preferable embodiment according to the invention, evenwhen an amplitude of an input signal is smaller than a power supplyvoltage, the setting operation is performed so as to assuredly turnon/off the first transistor. That is, in the setting operation, with thefirst switch turned on, a potential substantially equal to the firstinput potential is inputted to one terminal of the capacitor until thesecond transistor is turned off. Here, “the second transistor beingturned off” means being turned off substantially. That is, the secondtransistor is not necessarily turned off completely (that is, a currentthat flows in the second transistor does not necessarily become zerocompletely), but the current that flows in the second transistor hasonly to become sufficiently small. In such a setting operation, acurrent is flowed to a capacitor connected between the control terminaland the input terminal of the first transistor through the secondtransistor of which second terminal and control terminal are connectedto each other (that is, diode-connected) until the second transistor isturned off or a current value becomes very small. According to this, thecapacitor can be charged so that a voltage across thereof may be aproper voltage that reflects difference between the first power supplypotential and the first input potential and the threshold voltage of thefirst transistor. Thereby, in a normal operation, when a voltage of thecharged capacitor is added to the input signal followed by inputting tothe control terminal of the first transistor, the first transistor canbe assuredly turned on/off. The reason for the threshold voltage of thefirst transistor being able to reflect on the voltage of the capacitoris that the threshold voltage of the first transistor and that of thesecond transistor are substantially equal. The threshold voltages of thefirst and the second transistors, though desirably equal, may be alittle different as far as in the setting operation a capacitor for usein correction of input signal can be properly charged to allow operatinga digital circuit normally. Furthermore, in the case of an FET beingused as a transistor, the threshold voltage is plus for an N-type andminus for a P-type in many cases. However, even when the thresholdvoltage takes a value other than that, the invention can be applied.

Furthermore, a rectifier element is preferably connected in parallelwith the second transistor and so that the forward direction thereof maybe opposite to the forward direction of the second transistor. Thereby,even in the case of electric charges that oppositely bias thediode-connected first transistor being accumulated in the capacitorowing to, for instance, noise and so on, when the first switch is turnedon in a setting operation, a current is capable of flowing through therectifier element, and thereby a voltage across the capacitor can beconverged to a proper value. The rectifier element may be formed of, forinstance, a diode-connected transistor having the same conductivity typeas that of the second transistor.

Still furthermore, it is preferable that a node between the capacitorand the control terminal of the first transistor is connected through afurther switch to a potential different from the first power supplypotential, and the further switch is turned on prior to the settingoperation, such that a potential of the node can be set to apredetermined potential. Here, the predetermined potential is such apotential at which the second transistor is turned on owing to thedifference between the first power supply potential and thepredetermined potential, after the potential of the node is set at apredetermined potential, when the first switch is turned on in thesetting operation that is carried out with the further switchturned-off. According to this, even in the case of electric chargesbeing accumulated without being desired in the capacitor owing to, forinstance, noise and so on, when, prior to the setting operation, apotential of a node between the capacitor and the control terminal ofthe first transistor is set at a proper value, the setting operation canbe assuredly performed, and thereby a voltage across the capacitor canbe converged to a proper value corresponding to the difference betweenthe first power supply potential and the first input potential and thethreshold voltage of the first transistor. When the different potentialfrom the first power supply potential is set at a second power supplypotential, the different potential can be preferably supplied with ease.

Furthermore, one terminal of the capacitor may be connected through thesecond switch to an input terminal and through a third switch to apotential substantially equal to the first input potential, so that thesecond switch is turned on and the first and the third switches areturned off in a normal operation, whereas the second switch is turnedoff and the first and the third switches are turned on in the settingoperation. According to this, the setting operation can be easilycarried out only by switching the switch, without controlling the inputpotential. Furthermore, even in the case of, for instance, twotransistors different in the polarity being used as the firsttransistor, the setting operation of these transistors can besimultaneously carried out.

According to another preferable embodiment of the invention, provided isa digital circuit in which even in the case of, for instance, a powersupply voltage being low, and the power supply voltage being notsufficiently large with respect to the absolute value of the thresholdvoltage of the transistor, the setting operation can be carried out sothat a sufficient current may flow to the transistor to operate withhigh-speed. In such a digital circuit, the node between the capacitorand the control terminal of the first transistor is connected throughthe second switch to a predetermined potential. The setting operationincludes a first setting operation and a second setting operation. Inthe first setting operation, the second switch is turned on and thefirst input potential is inputted to the input terminal to charge thecapacitor. In the second setting operation, with the first inputpotential inputting to the input terminal, the second switch is turnedoff and the first switch is turned on, and thereby through the secondtransistor the capacitor is discharged. The discharge of the capacitorthrough the second transistor is carried out until a current flowing thesecond transistor becomes substantially zero, that is, until a voltageacross the capacitor may be substantially equal to the threshold voltageof the second transistor. The above-mentioned predetermined potential isa potential at which when the first switch is turned on in the secondsetting operation the second transistor is turned on, and can be set at,for instance, a second power supply potential different from the firstpower supply potential. Furthermore, typically, the first inputpotential is equal to the first power supply potential and the secondinput potential is equal to the second power supply potential.

When the voltage across the capacitor is set as mentioned above, in anormal operation, when the first input potential is inputted to theinput terminal, potential difference between the control terminal andthe first terminal of the first transistor becomes equal to thethreshold voltage of the first transistor to turn off the firsttransistor. Meanwhile, when the second input potential is inputted, thevoltage across the capacitor is superposed on the second input potentialso that the first transistor may be accelerated in turning on, andthereby a sufficient current can flow to the first transistor to turn onwith high-speed.

Furthermore, one terminal of the capacitor may be connected through thethird switch to the input terminal and connected through a fourth switchto a potential substantially equal to the first input potential, so thatthe third switch is turned on and the first, the second and the fourthswitches are turned off in a normal operation, the second and the fourthswitches are turned on and the third switch is turned off in the firstsetting operation, and the second and the third switches are turned offand the first and the fourth switches are turned on in the secondsetting operation. By thus setting, the setting operation can be easilycarried out only by switching the switches, without controlling theinput potential. Still furthermore, in the case of, for instance, twotransistors different in the polarity being used as the firsttransistor, the setting operation of these transistors can besimultaneously performed.

The switching circuit can take various forms such as an invertercircuit, a clocked inverter circuit, a logic circuit such as a NAND anda NOR, a level shift circuit or a transfer gate. In the case of theinverter circuit, the one using a transistor and a resistor, or the oneusing transistors with the same polarity one of which is diode-connectedso as to operate as a resistor can be used as well as a CMOS inverterusing two MOSFETs different in the polarity. In the case of the clockedinverter circuit, a transistor that provides a correcting circuit may beeither or both of a transistor that constitutes the inverter body and atransistor for use in clock signal synchronization.

The abovementioned switches (the first switch connected in series withthe diode-connected second transistor, and so on) may be any one ofelectrical ones and mechanical ones as far as a current flow can becontrolled. They may be transistors, diodes, or logic circuits made ofcombinations thereof. When the switches are made of semiconductorelements such as MOSFETs, it is preferable because an entire digitalcircuit can be formed through a semiconductor process. When the switchis made of a transistor, since it is used only as the switch, theconductivity type of the transistor is not particularly restricted.However, in the case of an off current being desirable to be small, itis desirable to use a transistor having the polarity less in the offcurrent. As a transistor less in the off current, there is the one inwhich an LDD region is disposed, and so on. Furthermore, when apotential of a source terminal of a transistor functioning as a switchoperates in a state close to a low potential side power supply (Vss,Vgnd, OV and so on), an n-channel type is desirably used. On thecontrary, when a potential of the source terminal operates in a stateclose to a high potential side power supply (Vdd and so on), a p-channeltype is desirably used. The reason for this is that since the absolutevalue of a gate-source voltage can be made larger, the transistor caneasily operate as a switch. With both an n-channel type and a p-channeltype, a CMOS type switch may be formed.

Furthermore, in order to inhibit electric charges accumulated owing tonoise and so on in the capacitor without being desired from adverselyaffecting during the setting operation, a further switch may beconnected in parallel with the capacitor. When the switch is turned onprior to the setting operation, the electric charges accumulated in thecapacitor can be discharged.

With the aforementioned digital circuit having the switching circuitusing transistors, various semiconductor devices (or electronicapparatuses) typified by integrated circuits and semiconductor displaydevices can be preferably realized. Such semiconductor devices include aliquid crystal display device, a self-light emitting type display devicehaving an organic EL display light emitting element in each pixel, a DMD(Digital Micromirror Device), a PDP (Plasma Display Panel), an FED(Field Emission Display) and the like, and the digital circuit accordingto the invention can be applied to a driver circuit and so on thereof.When the digital circuit according to the invention is applied to asemiconductor device that is formed by using a glass substrate, since anamplitude of a signal inputted from an IC is not needed to be controlledwith a booster circuit, a semiconductor device can be reduced in size,leading to lowered cost of the device itself.

The characteristics, objects and effects of the invention will be moreclarified when preferable embodiments are explained with reference tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a schematic configuration of theinvention.

FIG. 2 is a circuit diagram showing one embodiment of a digital circuitbased on the invention.

FIG. 3 a is a diagram showing a setting operation of the digital circuitshown in FIG. 2, and FIG. 3 b is a diagram showing a normal operation.

FIG. 4 is a circuit diagram showing another embodiment of a digitalcircuit based on the invention.

FIG. 5 is a circuit diagram showing another embodiment of a digitalcircuit based on the invention, that is formed by applying the inventionto a CMOS inverter circuit.

FIGS. 6 a and 6 b are diagrams each showing a setting operation of thedigital circuit shown in FIG. 5.

FIG. 7 is a circuit diagram of a digital circuit in which switches SW2and SW3 shown in FIG. 5 are formed of a PMOSFET 38 and an NMOSFET 39,respectively.

FIG. 8 is a circuit diagram showing another embodiment of the modifieddigital circuit shown in FIG. 5.

FIG. 9 is a circuit diagram showing another embodiment of the modifieddigital circuit shown in FIG. 5.

FIG. 10 is a circuit diagram showing still another embodiment of themodified digital circuit shown in FIG. 5.

FIGS. 11 a and 11 b are diagrams each showing an initializationoperation in the digital circuit shown in FIG. 10.

FIG. 12 is a circuit diagram showing a digital circuit in which theswitch shown in FIG. 10 is formed of a MOSFET.

FIG. 13 is a circuit diagram showing still another embodiment of themodified digital circuit shown in FIG. 5.

FIG. 14 is a circuit diagram showing one embodiment of a clockedinverter circuit to which the invention is applied.

FIG. 15 is a circuit diagram showing another embodiment of the modifiedclocked inverter circuit shown in FIG. 14.

FIG. 16 is a circuit diagram showing another embodiment of the modifiedclocked inverter circuit based on the invention shown in FIG. 14.

FIG. 17 is a diagram schematically showing an essential portion of adriver circuit of an active matrix device that is used in a liquidcrystal display device and so on and showing a typical unit circuit in ashift register of the driver circuit.

FIG. 18 is a circuit diagram showing an embodiment in which theinvention is applied to a clocked inverter on a left side in the unitcircuit of the shift register shown in FIG. 17.

FIG. 19 is a timing chart showing signals (potentials) of the respectiveportions in an initialization, a setting operation and a normaloperation of a shift register including the clocked inverter circuitshown in FIG. 18.

FIG. 20 is a circuit diagram showing another embodiment of the modifiedembodiment shown in FIG. 18.

FIG. 21 is a timing chart showing signals (potentials) of the respectiveportions in an initialization, a setting operation and a normaloperation of a shift register including the clocked inverter circuitshown in FIG. 20.

FIG. 22 is a circuit diagram showing another embodiment of the clockedinverter shown in FIG. 18.

FIG. 23 is a circuit diagram showing a typical unit circuit in a firstlatch circuit shown in FIG. 17.

FIG. 24 is a circuit diagram showing an embodiment in which theinvention is applied to the clocked inverter of the first latch circuitshown in FIG. 23.

FIG. 25 is a timing chart showing signals (potentials) of the respectiveportions in an initialization, a setting operation and a normaloperation of the clocked inverter shown in FIG. 24.

FIG. 26 a is a diagram schematically showing a return period in aselective period of one gate and FIG. 26 b is a diagram schematicallyshowing a driver stop period.

FIG. 27 is a circuit diagram showing an embodiment in which theinvention is applied to a transistor constituting a NAND circuit.

FIG. 28 is a circuit diagram showing an embodiment in which theinvention is applied to a transistor constituting a NOR circuit.

FIG. 29 is a circuit diagram showing still another embodiment of adigital circuit based on the invention.

FIGS. 30 a and 30 b are diagrams each showing a setting operation of thedigital circuit shown in FIG. 29.

FIGS. 31 a and 31 b are diagrams each showing a setting operation of thedigital circuit shown in FIG. 29.

FIGS. 32 a and 32 b are diagrams each showing a normal operation of thedigital circuit shown in FIG. 29.

FIG. 33 is a circuit diagram showing still another embodiment of adigital circuit based on the invention.

FIGS. 34 a and 34 b are diagrams each showing a setting operation of thedigital circuit shown in FIG. 33.

FIG. 35 is a circuit diagram showing a normal operation of the digitalcircuit shown in FIG. 33.

FIG. 36 a is a circuit diagram showing a typical example of an existingCMOS inverter circuit, and FIGS. 36 b and 36 c are diagrams each showinga normal operation of the CMOS inverter circuit shown in FIG. 36 a.

FIGS. 37 a and 37 b are diagrams for explaining a problem of the CMOSinverter circuit shown in FIG. 36.

FIGS. 38 a to 38 h are diagrams of electronic apparatuses to which theinvention can be applied.

BEST MODE FOR CARRYING OUT THE INVENTION

In what follows, most preferable embodiments according to the inventionwill be explained with reference to the drawings.

FIG. 1 is a block diagram showing a schematic configuration of a digitalcircuit based on the invention. As shown in FIG. 1, a digital circuit 1based on the invention includes a switching circuit 2 having atransistor such as a MOSFET, that is connected between an input terminalIN and an output terminal OUT, and outputs a different signal (forinstance, a high level power supply potential VDD or a low level powersupply potential VSS) to the output terminal in accordance with a valueof an input signal inputted to the input terminal; and a correctingcircuit 3 connected between the input terminal IN and the switchingcircuit 2.

FIG. 2 is a circuit diagram showing one embodiment of a digital circuitbased on the invention. A digital circuit 10 includes, as a switchingcircuit, an inverter circuit 12 that is constituted of one PMOSFET 11and a resistor R1. The PMOSFET 11 has a threshold voltage V_(THP), asource thereof being connected to a high level power supply potentialVDD, and a drain thereof being connected to a low level power supplypotential VSS (for instance, a ground potential V_(GND)) through theresistor R1. A gate that works as a control terminal of the PMOSFET 11is connected to an input terminal IN to which an input signal thatoscillates between a high level input potential V_(INH) and a low levelinput potential V_(INL) is inputted, a node N1 between the drain and theresistor R1 being connected to an output terminal OUT.

Between the gate of the PMOSFET 11 and the input terminal IN, acorrecting circuit 13 is connected. The correcting circuit 13 includes acapacitor C1 connected between the gate of the PMOSFET 11 and the inputterminal IN, a PMOSFET 14 that has the same conductivity and thesubstantially same threshold voltage V_(THP) as the PMOSFET 11, and aswitch SW1. A drain of the PMOSFET 14 is connected to a node N2 betweenthe capacitor C1 and the gate of the PMOSFET 11, a source thereof beingconnected through the switch SW1 to the high level power supplypotential VDD. The switch SW1 may be disposed between the drain of thePMOSFET 14 and the node N2, that is, has only to be connected in serieswith the PMOSFET 14. Furthermore, the PMOSFET 14, with the gate and thedrain connected to each other, forms a so-called “diode-connection”.Thereby, a gate-source voltage V_(GS) of the PMOSFET 14 becomes equal toa source-drain voltage V_(DS) thereof

An operation of thus constituted digital circuit 10 will be explainedbelow. For the sake of explanation, in this embodiment, it is assumedthat a high level input potential V_(INH) of an input signal inputted tothe input terminal IN is lower than a value obtained by subtracting theabsolute value of a threshold voltage |V_(THP)| from a high level powersupply potential VDD (that is, a value at which, in an existing circuit,when an input signal is a high level input potential V_(INH), thePMOSFET 11 is not turned off) and a low level input potential V_(INL) isequal to a ground potential V_(GND) (that is, a value low enough to turnon the PMOSFET 11).

Firstly, in the setting operation, as shown in FIG. 3 a, the switch SW1is turned on, and in this state, a high level input potential V_(INH) isinputted to the input terminal IN. Accordingly, a current flows throughthe PMOSFET 14 as shown by an arrow in the drawing and thereby thecapacitor C1 is charged. After a sufficient time, a voltage across thecapacitor C1 rises, thereby the absolute value of a gate-source voltageV_(GS) of the PMOSFET 14 becomes small, finally the PMOSFET 14 is turnedoff, and thus the current is stopped. At this time, a voltage across thecapacitor C1 becomes VDD−V_(INH)−|V_(THP)|.

After the capacitor C1 is properly charged thus in the settingoperation, in a normal operation, as shown in FIG. 3 b, the switch SW1is turned off, and an input signal that oscillates between a high levelinput potential V_(INH) and a low level input potential V_(INL) isinputted to the input terminal IN. Since the switch SW1 is turned off atthis time, electric charges accumulated in the capacitor C1 are held,that is, a voltage across the capacitor C1 is maintained constant.Accordingly, in the case of a high level input potential V_(TNH) beinginputted to the input terminal IN, since a voltage across the capacitorC1, VDD−V_(INH)−|V_(THP)|, is added thereto, a gate potential of thePMOSFET 11 becomes VDD−|V_(THP)|, and a gate-source voltage V_(GS)thereof becomes −|V_(THP)|. Accordingly, the PMOSFET 11 can be assuredlyturned off without causing a leakage current. Thereby, a groundpotential V_(GND) is outputted to the output terminal OUT. The settingoperation is not necessarily carried out until the PMOSFET 14 iscompletely turned off (that is, until the current flowing through thePMOSFET 14 becomes completely zero). Even when a slight current flows inthe PMOSFET 14, at a time when the capacitor C1 is charged enough to anextent that allows properly correcting an input signal in a normaloperation (that is, the PMOSFET 14 is substantially turned off), thesetting operation can be finished without causing practical operationalproblems.

On the other hand, in the case of a low level input potential V_(INL)being inputted to the input terminal IN, a gate potential of the PMOSFET11 becomes lower than that in the case of a high level input potentialV_(INH) being inputted to the input terminal IN, and V_(GS) becomes−|V_(THP)|−(V_(INH)−V_(INL)). Accordingly, V_(GS)<−|V_(THP)| issatisfied, the PMOSFET 11 is turned on, and a potential of the outputterminal OUT becomes substantially equal to the high level power supplypotential VDD. In the case of the capacitor C1 being not sufficientlylarge with respect to a gate capacitance of the PMOSFET 11, an inputvoltage (V_(INH), V_(INL)) is divided by the capacitor C1 and the gatecapacitance, therefore, a sufficient voltage is not applied to the gateof the PMOSFET 11. Accordingly, the amount of the capacitor C1 ispreferably determined in consideration of the gate capacitance of thePMOSFET 11 connected to the capacitor C1. For example, it is desirablethat the capacitor C1 has five times as large capacitance as the gatecapacitance of the PMOSFET 11.

As described above, in this embodiment, even in the case of a high levelinput potential V_(INH) being lower than a high level power supplypotential VDD as the first power supply potential, the capacitor C1connected between the gate of the PMOSFET 11 constituting the invertercircuit 12 and the input terminal IN is charged to a proper voltage, inthe setting operation, through the diode-connected PMOSFET 14 that hasthe substantially same threshold voltage as the PMOSFET 11 and is usedfor setting operation, and thereby the PMOSFET 11 can be assuredlyturned off. According to the invention, a booster is not needed to beprovided additionally, therefore, the cost reduction and downsizing of adevice can be achieved. Further, in the case of inputting a signal froman IC to a digital circuit formed on a glass substrate., the signal canbe inputted directly to the digital circuit without using a boostercircuit. Note that in the above embodiment, even when a high level inputpotential V_(INH) is equal to or higher than a high level power supplypotential VDD, the capacitor C1 can be operated normally in the settingoperation, though it is not charged.

In the case of a plurality of digital circuits 10 being used for a driveunit of a liquid crystal display or an organic EL display for instance,a plurality of PMOSFETs 11 constituting each inverter circuit 12 areincluded, and threshold voltages thereof may vary owing to variations inthe impurity concentration, the crystalline state of channel portions,and so on. According to the invention, however, a threshold voltage ofthe diode-connected PMOSFET 14 that is included in the correctingcircuit 13 corresponding to each PMOSFET 11 is substantially equal tothat of the PMOSFET 11 constituting the inverter circuit 12, and therebythe DC level converting capacitor C1 in the correcting circuit 13 can becharged so as to supply a proper voltage depending on the thresholdvoltage of the corresponding PMOSFET 11. In an actual semiconductorcircuit, these PMOSFETs 11 and 14 are provided close to each other so asnot to have differences in the impurity concentration and so on.According to this, the threshold voltage of the PMOSFET 11 constitutingthe inverter circuit 12 can be made substantially equal to that of thePMOSFET 14 for setting operation. Further, in the case of including amanufacturing step for crystallizing a channel portion by laserirradiation, channel portions of the PMOSFET 11 and the PMOSFET 14 arepreferably crystallized by laser beam spot with the same pulse in orderto make the threshold voltages closer to each other. It is preferablethat the sizes of the channel length L, the channel width W and so on ofthe PMOSFETs 11 and 14 are substantially same to easily realize thesubstantially same threshold voltage. However, the sizes of the PMOSFET11 and the PMOSFET 14 may be different as long as the threshold voltagesthereof are substantially the same. For example, the channel lengthand/or the channel width W of the PMOSFET 14 can be made small in orderto reduce the layout area. Alternatively, the channel width W of thePMOSFET 14 may be made larger so as to complete the setting operation ina shorter time.

In the above embodiment, the switch SW1 that is connected in series withthe diode-connected PMOSFET 14 is turned off in the normal operation,therefore, electric charges that are accumulated in the capacitor C1 ofthe correcting circuit 13 in the selling operation are held. Thus, thereis no concern of the capacitor C1 adversely affecting on the dynamiccharacteristics of the digital circuit 10 (that is, operation speed ofthe digital circuit 10 is not lowered) in the normal operation. On thecontrary, the capacitor C1, being connected in series with parasiticcapacitance generated between the gate and the drain or the source ofthe PMOSFET 11 to lower total capacitance, can contribute to improve thedynamic characteristics. Furthermore, since the setting operation iscarried out before the normal operation is not carried out assuredly dueto leakage of electric charges accumulated in the capacitor C1, there isno need to frequently carry out the setting operation, and powerconsumption due to the setting operation is thus only slight. In acircuit connected to an input side of the digital circuit 10, anoperating voltage (power supply voltage and a signal voltage) can belowered, which also contributes to suppress power consumption.

FIG. 4 is a circuit diagram showing another embodiment of the digitalcircuit based on the invention, which includes a level shift circuitusing one PMOSFET as a switching circuit. In this drawing, the sameportions as FIG. 2 are denoted by the same reference numerals andexplained in no more details. A digital circuit 20 shown in FIG. 4 hasthe substantially same configuration as the digital circuit 10 shown inFIG. 2, except that the drain of the PMOSFET 11 is connected to a groundpotential V_(GND) as a low level power supply potential VSS, the sourcethereof is connected to a high level power supply potential VDD throughthe resistor R1, the output terminal OUT is connected to the node N3between the source of the PMOSFET 11 and the resistor R1, and thereby alevel shift circuit 21 is formed as a switching circuit. Although theexplanation is omitted here, in this embodiment, the capacitor C1 isproperly charged by a similar setting operation as the above embodiment,and thereby the PMOSFET 11 can be assuredly turned on/off withoutintroducing errors in the normal operation. In this embodiment, when ahigh level input potential V_(INH) is input to the input terminal IN,the PMOSFET 11 is turned off and a high level power supply potential VDDis outputted to the output terminal OUT, whereas when a low level inputpotential V_(INL) is input, the PMOSFET 11 is turned on and a low levelpower supply potential VSS is outputted to the output terminal OUT. Asdescribed above, a switching circuit that supplies different signals tothe output terminal OUT in accordance with the ON/OFF state of atransistor may include various embodiments, and the invention can beapplied to such a switching circuit in order to assuredly turn on/off atransistor included therein.

FIG. 5 is a circuit diagram showing a CMOS inverter circuit to which theinvention is applied as still another embodiment of the digital circuitbased on the invention. A digital circuit 30 comprises a CMOS invertercircuit 31 as a switching circuit. The CMOS inverter circuit 31, asever, includes a PMOSFET 32 having a threshold voltage V_(THP) and anNMOSFET 33 having a threshold voltage V_(THN) that are connected inseries between a high level power supply potential VDD as a power supplypotential and a low level power supply potential VSS. A source of thePMOSFET 32 is connected to the high level power supply potential VDD anda source of the NMOSFET 33 is connected to the low level power supplypotential VSS (ground potential V_(GND) in this example). Drains of theMOSFETs 32 and 33 are connected to each other, and a connection point(node) N4 thereof is connected to an output terminal OUT. Both gates ofthe MOSFETs 32 and 33 are connected to an input terminal IN to which aninput signal that oscillates between a high level input potentialV_(INH) and a low level input potential V_(INL) is inputted.

According to the invention, a correcting circuit 34 is connected betweenthe gate of the PMOSFET 32 and the input terminal IN. The correctingcircuit 34 includes, similarly to the correcting circuit 13 shown inembodiment of FIG. 2, a capacitor C2 connected between the gate of thePMOSFET 32 and the input terminal IN, a PMOSFET 35 for setting operationthat has the same conductivity and the substantially same thresholdvoltage V_(THP) as the PMOSFET 32, and a switch SW2. A drain of thePMOSFET 35 is connected to a node N5 between the capacitor C2 and thegate of the PMOSFET 32, a source thereof being connected through theswitch SW2 to the high level power supply potential VDD. Furthermore,the PMOSFET 35, with the gate and the drain connected to each other,forms a diode-connection. The switch SW2 is connected in series with thePMOSFET 35 similarly to in FIG. 2.

A correcting circuit 36 is connected between the gate of the NMOSFET 33and the input terminal IN. The correcting circuit 36 includes acapacitor C3 connected between the gate of the NMOSFET 33 and the inputterminal IN, an NMOSFET 37 for setting operation tat has the sameconductivity and the substantially same threshold voltage V_(THN) as theNMOSFET 33, and a switch SW3. A drain of the NMOSFET 37 is connected toa node N6 between the capacitor C3 and the gate of the NMOSFET 33, asource thereof being connected through the switch SW3 to the low levelpower supply potential VSS. Furthermore, the NMOSFET 37, with the gateand the drain connected to each other, forms a diode-connection. Theswitch SW3 may be disposed between the NMOSFET 37 and the low levelpower supply potential VSS.

An operation of thus constituted digital circuit 30 will be explainedbelow with reference to FIG. 6. For the sake of explanation, it isassumed that a high level input potential V_(INH) of an input signalinputted to the input terminal IN is lower than a value obtained bysubtracting the absolute value of a threshold voltage |V_(THP)| of thePMOSFET 32 from the VDD, and a low level input potential V_(INL) ishigher than a value obtained by adding an absolute value of thethreshold voltage of the NMOSFET 33 |V_(THL)| to the low level powersupply potential VSS (V_(GND)).

As shown in FIG. 6 a, the switch SW2 is turned on and the switch SW3 isturned off and in this state, a high level input potential V_(INH) isinputted to the input terminal IN. Accordingly, a current flows throughthe diode-connected PMOSFET 35 as shown by an arrow, and thereby thecapacitor C2 connected to the gate of the PMOSFET 32 is charged. When avoltage across the capacitor C2 becomes equal to VDD−V_(INH)−|V_(THP)|,the PMOSFET 35 is turned off and the current is stopped (P-channelsetting operation). Subsequently, as shown in FIG. 6 b, the switch SW2is turned off and the switch SW3 is turned on, and in this state, a lowlevel input potential V_(INL) is inputted to the input terminal IN.Accordingly, a current flows through the diode-connected NMOSFET 37 asshown by an arrow, and thereby the capacitor C3 connected to the gate ofthe NMOSFET 33 is charged. When a voltage across the capacitor C3becomes equal to VSS−V_(INL)+|V_(THN)|, the NMOSFET 37 is turned off andthe current is stopped (N-channel setting operation).

After the capacitors C2 and C3 are properly charged thus in the settingoperation, in a normal operation, the switches SW2 and SW3 are turnedoff, and an pulsed input signal that oscillates between a high levelinput potential V_(INH) and a low level input potential V_(INL) isinputted to the input terminal IN. Since the switches SW2 and SW3 areturned off at this time, electric charges accumulated in the capacitorsC2 and C3 are held, that is, a voltage across the capacitors C2 and C3is maintained constant. In the case of a high level input potentialV_(INH) being inputted to the input terminal IN, a gate potential of thePMOSFET 32 becomes equal to VDD−|V_(THP)|, and a gate-source voltageV_(GS) thereof becomes equal to −|V_(THP)|, and thereby the PMOSFET 32can be turned off. Since the NMOSFET 33 is on at this time, a low levelpower supply potential VSS (ground potential V_(GND)) is outputted tothe output terminal OUT. On the other hand, in the case of a low levelinput potential V_(INL) being inputted to the input terminal IN, a gatepotential of the NMOSFET 33 becomes equal to VSS+|V_(THN)|, and agate-source voltage V_(GS) thereof becomes equal to |V_(THN)|, andthereby the NMOSFET 33 can be turned off. Since the PMOSFET 32 is on atthis time, a high level power supply potential VDD is outputted to theoutput terminal OUT. The setting operations are not necessarily carriedout until the PMOSFET 35 and the NMOSFET 37 are completely turned off.At a time when the current flowing in the MOSFETs 35 and 37 issufficiently small, (that is, the MOSFETs 35 and 37 are substantiallyturned off), the setting operations can be finished. It is needless tosay that although the setting operation of the PMOSFET 35 is followed bythe setting operation of the NMOSFET 37 in the above embodiment, theorder is not limited to this and the setting operation of the NMOSFET 37may be carried out firstly.

As described above, when the invention is applied to the pair of PMOSFET32 and NMOSFET 33 constituting the CMOS inverter circuit 31, even in thecase of a high level input potential V_(INH) being lower than a highlevel power supply potential VDD and a low level input potential V_(INL)being higher than a low level power supply potential VSS, the capacitorsC2 and C3 connected between the gates of the PMOSFET 32 and the NMOSFET33 and the input terminal IN are charged to a proper voltage, in thesetting operation, in accordance with differences between the thresholdvoltages of the MOSFETs 32 and 33, and the input potentials V_(INH) andV_(INL) and the power supply potentials VDD and VSS. Thus, the PMOSFET32 and the NMOSFET 33 can be assuredly turned on/off and a propercircuit operation can be realized.

FIG. 7 shows a circuit diagram of the digital circuit 30 in which theswitches SW2 and SW3 shown in FIG. 5 are formed of a PMOSFET 38 and anNMOSFET 39 respectively. In this drawing, the same portions as FIG. 5are denoted by the same reference numerals. Gates of the PMOSFET 38 andthe NMOSFET 39 are connected to a P-channel control signal line 40 andan N-channel control signal line 41 respectively. In a P-channel settingoperation, potentials of the control signal lines 40 and 41 are equal toa low level power supply potential VSS for instance, and a low levelpower supply potential VSS is inputted to the gates of the PMOSFET 38and the NMOSFET 39, and thereby the PMOSFET 38 is turned on while theNMOSFET 39 is turned off, further, a high level input potential V_(INH)is inputted to the input terminal IN. In an N-channel setting operation,potentials of the control signal lines 40 and 41 are equal to a highlevel power supply potential VDD for instance, and a high level powersupply potential is inputted to the gates of the PMOSFET 38 and theNMOSFET 39, and thereby the PMOSFET 38 is turned off while the NMOSFET39 is turned on, further, a low level input potential V_(INL) isinputted to the input terminal IN. According to such setting operations,as described with reference to FIG. 6 a and FIG. 6 b, electric chargesare properly accumulated in the capacitors C2 and C3. In a normaloperation, a potential of the P-channel control signal line 40 is equalto the high level power supply potential VDD whereas a potential of theN-channel control signal line 41 is equal to the low level power supplypotential VSS, and both of the PMOSFET 38 and the NMOSFET 39 are turnedoff.

The capacitors C2 and C3, as shown by a magnified view in FIG. 7, can beformed by using capacitance generated between a gate and a source and/ora drain of one or a plurality of MOSFETs. In the case of connecting aMOSFET used as capacitance, it may be connected in such a direction thatthe MOSFET is turned on (that is, a channel is formed) when it ischarged. For instance, in the case of one PMOSFET being connected as thecapacitor C2 shown in FIG. 7, a gate side terminal may be connected tothe input terminal IN whereas a source/drain side terminal may beconnected to the gate of the PMOSFET 32. The conductivity of a MOSFETused as a capacitor may be either an N-type or a P-type, though thethreshold voltage thereof is preferably close to zero.

In the aforementioned digital circuit 30, it is assumed that electriccharges are not accumulated in the capacitors C2 and C3 before a settingoperation. However, electric charges may be accumulated in thecapacitors C2 and C3 owing to, for instance, noise and so on. In thecase of, due to such electric charges accumulated in the capacitors C2and C3, the capacitors C2 and C3 is excessively charged with thepolarity shown in FIG. 6 b before a setting operation, even when theswitches SW2 and SW3 are turned on in the setting operation, thediode-connected MOSFETs 35 and 37 are not turned on, and the electriccharges accumulated in the capacitors C2 and C3 (namely, a voltageacross the capacitors C2 and C3) are held without change, and therebythe voltage across the capacitors C2 and C3 (or a gate potential of theMOSFETs 32 and 33) may not be converged to a proper value. Thus, somemeasures are preferably taken in order to set the voltage across thecapacitors C2 and C3 to a proper value even when such undesired electriccharges are accumulated in the capacitors C2 and C3.

FIG. 8 is a circuit diagram showing another embodiment of the modifieddigital circuit 30 shown in FIG. 5. In this drawing, the same portionsas FIG. 5 are denoted by the same reference numerals and are explainedin no more details. In a digital circuit 30 a, another diode-connectedPMOSFET 42 is connected in parallel with the diode-connected PMOSFET 35so that the forward direction thereof is opposite to that of the PMOSFET35. Similarly, another diode-connected NMOSFET 43 is connected inparallel with the diode-connected NMOSFET 37 so that the forwarddirection thereof is opposite to that of the NMOSFET 37. According tothis, in the case of electric charges that can oppositely bias thediode-connected PMOSFET 35 and NMOSFET 37 being accumulated in thecapacitors C2 and C3 before a setting operation owing to, for instance,noise and so on, when the switches SW2 and SW3 are turned on in thesetting operation, a current is capable of flowing as shown by an arrowin FIG. 8, and thereby a voltage across the capacitors C2 and C3 can beconverged to a substantially proper value. When threshold voltages ofthe diode-connected MOSFETs 42 and 43 are equal to threshold voltagesV_(THP) and V_(THN) of the MOSFETs 32 and 33 respectively, a gatepotential of the PMOSFET 32 (that is, a potential of a node N5) isconverged to a potential expressed by VDD−|V_(THP)| while a gatepotential of the NMOSFET 33 (that is, a potential of a node N6) isconverged to a potential expressed by VSS−V_(THN)|. Another rectifierelement such as a diode can be used instead of the diode-connectedMOSFETs 42 and 43. The diode-connected MOSFET 42 that is connected inparallel with the PMOSFET 35 may be an N-type. Further, thediode-connected MOSFET 43 that is connected in parallel with the NMOSFET37 may be a P-type.

FIG. 9 is a circuit diagram showing another embodiment of the modifieddigital circuit 30 shown in FIG. 5. In this drawing, the same portionsas FIG. 5 are denoted by the same reference numerals and are explainedin no more details. In a digital circuit 30 b, switches SW4 and SW5 areprovided in parallel with the capacitors C2 and C3 respectively.According to this, in the case of undesired electric charges beingaccumulated in the capacitors C2 and C3 owing to, for instance, noiseand so on, the switches SW4 and SW5 are turned on before a settingoperation, and thereby the capacitors C2 and C3 can be discharged. Thus,when the switches SW2 and SW3 are turned on in the setting operation,the diode-connected MOSFETs 35 and 37 are turned on assuredly, andthereby the capacitors C2 and C3 are properly charged.

FIG. 10 is a circuit diagram showing still another embodiment of themodified digital circuit 30 shown in FIG. 5. In this drawing, the sameportions as FIG. 5 are denoted by the same reference numerals and areexplained in no more details. In a digital circuit 30 c, the node N5between the gate of the PMOSFET 32 and the capacitor C2 is connectedthrough a switch SW6 to a low level power supply potential VSS, whilethe node N6 between the gate of the NMOSFET 33 and the capacitor C3 isconnected through a switch SW7 to a high level power supply potentialVDD.

As shown in FIG. 11 a, when the switch SW6 is turned on in aninitialization operation before a setting operation of the capacitor C2connected to the gate of the PMOSFET 32 (P-channel setting operation),even in the case of unnecessary electric charges being accumulated inthe capacitor C2 owing to, for instance, noise and so on, and apotential of the node N5 between the gate of the PMOSFET 32 and thecapacitor C2 being undesirably high, a potential of the node N5 can belowered to substantially equal the low level power supply potential VSS.A potential of the input terminal IN is preferably a high level inputpotential at this time, though it may be a low level input potential.Further, the switch SW2 may be either on state or off. However, when itis on, a current flows as shown by a dashed arrow in the drawing, andthereby a potential of the node N5 cannot be sufficiently lowered withease, therefore, the switch SW2 is preferably off.

Similarly, as shown in FIG. 1 b, when the switch SW7 is turned on in aninitialization operation before a setting operation of the capacitor C3connected to the gate of the NMOSFET 33 (N-channel setting operation),even in the case of unnecessary electric charges being accumulated inthe capacitor C3 owing to, for instance, noise and so on, and apotential of the node N6 between the gate of the NMOSFET 33 and thecapacitor C3 being undesirably low, a potential of the node N6 can beincreased to substantially equal the high level power supply potentialVDD. A potential of the input terminal IN is preferably a low levelinput potential at this time, though it may be a high level inputpotential. Further, the switch SW3 may be either on or off. However,when it is on, a current flows as shown by a dashed arrow in thedrawing, and thereby a potential of the node N6 cannot be sufficientlyincreased with ease, therefore, the switch SW3 is preferably off.

In a setting operation, the switches SW6 and SW7 are turned off, and asdescribed with reference to FIG. 6 a and FIG. 6 b, the switch SW2 or SW3is turned on. According to the aforementioned initialization operation,potentials of the nodes N5 and N6 are set to a proper value before thesetting operation. As a result, when the switches SW2 and SW3 are turnedon in the setting operation, the diode-connected MOSFETs 35 and 37 arebiased in the forward direction to be turned on assuredly, and a currentflows through the MOSFETs 35 and 37, and thereby the capacitors C2 andC3 can be properly charged. Although in the embodiments shown in FIG. 10and FIG. 11, the node N5 is connected to the low level power supplypotential VSS whereas the node N6 is connected to the high level powersupply potential VDD in the initialization operation, they may beconnected to another potential other than the power supply potential aslong as the diode-connected MOSFETs 35 and 37 are biased in the forwarddirection and turned on in the setting operation after theinitialization operation. However, since the power supply potential canbe obtained easily, it may be preferably used. Furthermore, in the aboveembodiments, a P-channel initialization operation and an N-channelinitialization operation are carried out separately, though they may becarried out simultaneously by turning on the switches SW6 and SW7 at thesame time.

FIG. 12 is a circuit diagram showing the digital circuit 30 c in whichthe switches SW2, SW3, SW6, and SW7 shown in FIG. 10 are formed ofMOSFETs 44, 45, 46, and 47. The MOSFET 44 is a PMOSFET whose gate isconnected to a P-channel control signal line 48. The MOSFET 45 is anNMOSFET whose gate is connected to an N-channel control signal line 49.The MOSFET 46 is an NMOSFET whose gate is connected to a P-channelinitialization signal line 50. The MOSFET 47 is a PMOSFET whose gate isconnected to an N-channel initialization signal line 51. When potentialsof the control signal lines 48 and 49 and the initialization signallines 50 and 51 being controlled properly, the MOSFETs 44 to 47 areproperly turned on/off, and thereby the initialization, setting, andnormal operations described above can be carried out. In this manner,each switch can be formed of a proper semiconductor element.

FIG. 13 is a circuit diagram showing still another embodiment of themodified digital circuit 30 shown in FIG. 5. In this drawing, the sameportions as FIG. 5 are denoted by the same reference numerals and areexplained in no more details. In a digital circuit 30 d, a terminal ofthe capacitor C2 on the opposite side to a terminal that is connected tothe gate of the PMOSFET 32 is connected through a switch SW8 to theinput terminal IN while connected through a switch SW9 to a potentialV_(H) that is substantially equal to a high level input potentialV_(INH) of an input signal inputted to the input terminal IN in a normaloperation. Similarly, a terminal of the capacitor C3 on the oppositeside to a terminal that is connected to the gate of the NMOSFET 33 isconnected through a switch SW10 to the input terminal IN while connectedthrough a switch SW11 to a potential V_(L) that is substantially equalto a low level input potential V_(INL) of an input signal inputted tothe input terminal IN in a normal operation.

In this embodiment, the switches SW2, SW3, SW9, and SW11 are turned onwhereas the switches SW8 and SW10 are turned off, and thereby a settingoperation of the capacitors C2 and C3 can be carried out at the sametime and independent of a potential of the input terminal IN. In anormal operation, the switches SW2, SW3, SW9, and SW11 are turned offwhereas the switches SW8 and SW10 are turned on, and inputted to theinput terminal IN is an input signal which oscillates between a highlevel input potential V_(INH) and a low level input potential V_(INL).

In a CMOS inverter, when a MOSFET is connected in series with a PMOSFETand an NMOSFET that constitute the inverter, and these MOSFETs areturned on/off by a clock signal (or a synchronizing signal having theopposite phase thereto such as an inverted clock signal), an output ofthe inverter is synchronized with the synchronizing signal such as theclock signal. Such an inverter is referred to as a clocked inverter. Theinvention can also be applied to a MOSFET for clock signalsynchronization connected in series with a PMOSFET and an NMOSFET thatconstitute a CMOS inverter. An embodiment thereof is shown in FIG. 14.

A clocked inverter circuit (digital circuit) 60 shown in FIG. 14comprises a PMOSFET 61 and an NMOSFET 62 that constitute a CMOSinverter. Gates of the MOSFETs 61 and 62 are connected to an inputterminal IN and a common drain thereof is connected to an outputterminal OUT. A source of the PMOSFET 61 is connected to a high levelpower supply potential VDD through a PMOSFET 63 for clocksynchronization, and a source of the NMOSFET 62 is connected to a lowlevel power supply potential VSS (ground potential V_(GND) in thisexample) through an NMOSFET 64 for clock synchronization. A gate of thePMOSFET 63 is connected to an inverted clock signal line 65 forsupplying an inverted clock signal whereas a gate of the NMOSFET 64 isconnected to a clock signal line 66 for supplying a clock signal. Theclock signal and the inverted clock signal oscillate between a highlevel potential V_(CH) lower than a high level power supply potentialVDD and a low level potential V_(CL) higher than a low level powersupply potential VSS. In this embodiment, an input signal inputted tothe input terminal IN oscillates between the high level power supplypotential VDD and the low level power supply potential VSS. However, inthe case of an amplitude of the input signal being small, similarly tothe embodiment described above, a correcting circuit can be provided forthe MOSFETs 61 and 62 that constitute the inverter. It is to be notedthat the PMOSFET 61 may be connected between the PMOSFET 63 and thepower supply potential VDD, and the NMOSFET 62 may be connected betweenthe NMOSFET 64 and the power supply potential VSS.

A correcting circuit 67 based on the invention is connected between thegate of the PMOSFET 63 and the inverted clock signal line 65. Thecorrecting circuit 67 includes a capacitor C4 connected between the gateof the PMOSFET 63 and the inverted clock signal line 65, adiode-connected PMOSFET 68 that has the substantially same thresholdvoltage as the PMOSFET 63, and a switch SW 12. A drain of the PMOSFET 68is connected to a node N7 between the capacitor C4 and the gate of thePMOSFET 63, a source thereof being connected through the switch SW12 tothe high level power supply potential VDD.

Similarly, a correcting circuit 69 is connected between the gate of theNMOSFET 64 and the clock signal line 66. The correcting circuit 69includes a capacitor C5 connected between the gate of the NMOSFET 64 andthe clock signal line 66, a diode-connected NMOSFET 70 that has thesubstantially same threshold voltage as the NMOSFET 64, and a switchSW13. A drain of the NMOSFET 70 is connected to a node N8 between thecapacitor C5 and the gate of the NMOSFET 64, a source thereof beingconnected through the switch SW13 to the low level power supplypotential VSS.

In this embodiment, a clock signal and an inverted clock signal can beconsidered as input signals in the invention when seen from thecorresponding MOSFETs 63 and 64. Furthermore, the PMOSFET 63 and thecorrecting circuit 67 or the NMOSFET 64 and the correcting circuit 69can be considered to form the digital circuit of the invention. In thatcase, the drains of the PMOSFET 63 and the NMOSFET 64 can be consideredas output terminals.

Firstly, in a setting operation, both the switches SW12 and SW13 areturned on, and in this state, a high level potential V_(CH) is inputtedas an inverted clock signal (at this time, a clock signal becomes a lowlevel potential V_(CL)). Since the high level potential V_(CH) is lowerthan a high level power supply potential VDD, the diode-connectedPMOSFET 68 is biased in the forward direction and turned on, and therebya current flows and the capacitor C4 is charged. The current flows untila voltage across the capacitor C4 becomes high enough to turn off thePMOSFET 68. Further at this time, a low level potential V_(CL) higherthan a low level power supply potential VSS is inputted as a clocksignal. Therefore, the diode-connected NMOSFET 70 is biased in theforward direction and turned on, and thereby a current flows and thecapacitor C5 is charged. After a voltage across the capacitor C5 risessufficiently, the NMOSFET 70 is turned off, and thus the current isstopped. As set forth above, in this embodiment, the setting operationsof the capacitors C4 and C5 in the two correcting circuits 67 and 69 canbe carried out at the same time.

In a normal operation, both the switches SW12 and SW13 are turned offand a clock signal, an inverted clock signal and an input signal areinputted. In this case also, the capacitors C4 and C5 are charged to aproper voltage corresponding to threshold voltages of the PMOSFET 63 andthe NMOSFET 64, and the clock signal and the inverted clock signal arebiased properly and inputted to the gates of the PMOSFET 63 and theNMOSFET 64. Therefore, the PMOSFET 63 and the NMOSFET 64 are assuredlyturned on/off, and an output signal can be synchronized with the clocksignal.

FIG. 15 is a circuit diagram showing another embodiment of the modifiedclocked inverter circuit 60 shown in FIG. 14. In this drawing, the sameportions as FIG. 14 are denoted by the same reference numerals and areexplained in no more details. A clocked inverter circuit 60 a shown inFIG. 15 comprises, similarly to the embodiment shown in FIG. 10,switches SW14 and SW15 that selectively connect nodes N7 and N8 betweenthe capacitors C4 and C5 and the corresponding gates of the MOSFETs 63and 64 to a low level power supply potential VSS and a high level powersupply potential VDD. According to this, the capacitors C4 and C5 forcorrection can be initialized by turning on the switches SW14 and SW15before a setting operation, and thereby even when undesired electriccharges being accumulated in the capacitors C4 and C5 owing to noise andso on, the MOSFETs 68 and 70 are not adversely affected by the electriccharges.

FIG. 16 is a circuit diagram showing another embodiment of the modifiedclocked inverter circuit 60 based on the invention shown in FIG. 14. Inthis drawing, the same portions as FIG. 14 are denoted by the samereference numerals and are explained in no more details. In a clockedinverter circuit 60 b shown in FIG. 16, similarly to the embodimentshown in FIG. 13, a terminal of the capacitor C4 on the opposite side toa terminal that is connected to the gate of the PMOSFET 63 is connectedthrough a switch SW16 to the inverted clock signal line 65 whileconnected through a switch SW17 to a potential V′_(H) that issubstantially equal to a high level potential V_(CH) of and invertedclock signal. Similarly, a terminal of the capacitor C5 on the oppositeside to a terminal that is connected to the gate of the NMOSFET 64 isconnected through a switch SW18 to the clock signal line 66 whileconnected through a switch SW19 to a potential V′_(L) that issubstantially equal to a low level potential V_(CL) of a clock signal.

In this embodiment, the switches SW12, SW13, SW17, and SW19 are turnedon whereas the switches SW16 and SW18 are turned off, and therebysetting operations of the capacitors C4 and C5 can be carried out at thesame time and independent of potentials of a clock signal and aninverted clock signal. In a normal operation, the switches SW12, SW13,SW17, and SW19 are turned off whereas the switches SW16 and SW18 areturned on. In this state, a clock signal and an inverted clock signalare inputted through the capacitors C4 and C5 to the gates of thePMOSFET 63 and the NMOSFET 64, and an input signal which oscillatesbetween a high level input potential V_(INH) and a low level inputpotential V_(INL) is inputted to the input terminal IN.

FIG. 17 is a diagram schematically showing an essential portion of adriver circuit of an active matrix device that is used in a liquidcrystal display, an organic EL display and showing a typical unitcircuit in a shift register of the driver circuit. A driver circuit 80comprises a shift register 81 for sequentially outputting a selectivesignal in synchronism with a clock signal and an inverted clock signal,a first latch circuit 82 for latching a video signal in accordance withthe selective signal from the shift register 81, and a second latchcircuit 83 for latching data transferred from the first latch circuit82. The shift register 81 comprises a plurality of unit circuits 84.Each of the unit circuits 84 includes two clocked inverters 85 and 86and one inverter 87, and is operated, for instance, so as to take aninput signal in a unit circuit 84 when a clock signal becomes a highlevel potential V_(CH) (at this time, an output signal may vary), andhold an output signal in a unit circuit 84 when a clock signal becomes alow level. In one unit circuit 84 and an adjacent unit circuit 84, aclock signal and an inverted clock signal are inverted. Therefore, whenan input signal is taken in one unit circuit 84, an output signal isheld in an adjacent unit circuit 84, and when an output signal is heldin one unit circuit 84, an input signal is taken in an adjacent unitcircuit 84. The configuration and operation of such shift register 81are well known in this field. An amplitude of a clock signal (or aninverted clock signal) inputted to the clocked inverters 85 and 86 ofthe shift register 81 is made smaller than a power supply voltage (highlevel power supply potential VDD—low level power supply potential VSS).In that case, some measures are preferably taken in order to assuredlyturn off these clocked inverters 85 and 86 without introducing errors.When the invention is applied to the clocked inverters 85 and 86, such aproblem can be solved without lowering operation speed.

FIG. 18 is a circuit diagram showing an embodiment in which theinvention is applied to the clocked inverter 85 on a left side in theunit circuit 84 of the shift register 81 shown in FIG. 17. In thisdrawing, the other clocked inverter 86 and the inverter 87 are notshown.

A clocked inverter 85 a on a left side of FIG. 18 (corresponding to theclocked inverter 85 on the left side in the unit circuit 84 of FIG. 17)comprises a PMOSFET 91 and an NMOSFET 92 whose drains are connected inseries with each other to constitute a CMOS inverter. The PMOSFET 91 isconnected through a PMOSFET 93 for clock synchronization to a high levelpower supply potential VDD, the NMOSFET 92 being connected through anNMOSFET 94 for clock synchronization to a low level power supplypotential VSS (for instance, V_(GND)).

A gate of the PMOSFET 93 is connected through a correcting circuit 97 toan inverted clock signal line 95, a gate of the NMOSFET 94 beingconnected through a correcting circuit 98 to a clock signal line 96. Thecorrecting circuit 97 includes a capacitor C6 connected between the gateof the PMOSFET 93 and the inverted clock signal line 95, adiode-connected PMOSFET 99 that has the substantially same thresholdvoltage as the PMOSFET 93, and a PMOSFET 100 that functions as a switchfor selectively carrying out a setting operation. The PMOSFET 99 and thePMOSFET 100 are connected in series between a node N9 between thecapacitor C6 and the gate of the PMOSFET 93 and a high level powersupply potential VDD. Similarly, the correcting circuit 98 includes acapacitor C7 connected between the gate of the NMOSFET 94 and the clocksignal line 96, a diode-connected NMOSFET 101 that has the substantiallysame threshold voltage as the NMOSFET 94, and an NMOSFET 102 thatfunctions as a switch for selectively carrying out a setting operation.The NMOSFET 101 and the NMOSFET 102 are connected in series between anode N10 between the capacitor C7 and the gate of the NMOSFET 94 and alow level power supply potential VSS. A gate of the PMOSFET 100 isconnected through an inverter 103 to a first control signal line 104, agate of the NMOSFET 102 being connected directly to the first controlsignal line 104.

Furthermore, the node N9 between the capacitor C6 and the gate of thePMOSFET 93 is connected through an NMOSFET 106 to the low level powersupply potential VSS, the node N10 between the capacitor C7 and the gateof the NMOSFET 94 being connected through a PMOSFET 107 to the highlevel power supply potential VDD. The capacitors C6 and C7 can beinitialized by selectively turning on/off the NMOSFET 106 and thePMOSFET 107. A gate of the NMOSFET 106 is connected directly to aninitialization signal line 108, a gate of the PMOSFET 107 beingconnected through an inverter 109 to the initialization signal line 108,and a signal with opposite polarity is inputted to each gate of theMOSFETs 106 and 107.

A clocked inverter 85 b on a right side of FIG. 18 (corresponding to theclocked inverter 85 on the right side in the unit circuit 84 of FIG. 17)has the same configuration as the clocked inverter 85 a on the leftside, except that the gate of the PMOSFET 93 is connected through thecapacitor C6 to the clock signal line 96, the gate of the NMOSFET 94 isconnected through the capacitor C7 to the inverted clock signal line 95,and the gates of the PMOSFET 100 and the NMOSFET 102 are connected to asecond control signal line 105. It is to be noted that although only thetwo clocked inverters 85 a and 85 b are shown in FIG. 18, a plurality ofthese inverters are arranged alternately in the actual circuit.

FIG. 19 is a timing chart showing preferable changes in signals(potentials) of the respective portions in an initialization, a settingoperation and a normal operation of thus constituted clocked inverters85 a and 85 b of the shift register 81.

In an initialization operation, a potential of the initialization signalline 108 becomes high, when a potential of the clock signal line 96 ishigh, a potential of the inverted clock signal line 95 is low, andpotentials of the first control signal line 104 and the second controlsignal line 105 are low. According to this, the NMOSFET 106 and thePMOSFET 107 in each of the clocked inverters 85 a and 85 b are turnedon, and the capacitors C6 and C7 in the correcting circuits 97 and 98are initialized. When the potential of the initialization signal line108 becomes a low level, the initialization operation is completed. Inthis embodiment, the initialization operation is simultaneously carriedout in the clocked inverter 85 a on the left side and the clockedinverter 85 b on the right side. Therefore, in the initializationoperation, in the clocked inverter 85 b on one side (the right side inthis example), a high level potential V_(CH) is inputted to thecapacitor C6 connected to the gate of the PMOSFET 93 while a low levelpotential V_(CL) is inputted to the capacitor C7 connected to the gateof the NMOSFET 94. Meanwhile, in the clocked inverter 85 a on the otherside (the left side in this example), a low level potential V_(CL) isinputted to the capacitor C6 connected to the gate of the PMOSFET 93while a high level potential V_(CH) is inputted to the capacitor C7connected to the gate of the NMOSFET 94.

A setting operation is composed of a first setting operation in whichelectric charges are accumulated in the capacitors C6 and C7 in theclocked inverter 85 a on the left side of FIG. 18 and a second settingoperation in which electric charges are accumulated in the capacitors C6and C7 in the clocked inverter 85 b on the right side of FIG. 18. In thefirst setting operation, in a first phase, potentials of the firstcontrol signal line 104 and the inverted clock signal line 95 become ahigh level while potentials of the second control signal line 105 andthe clock signal line 96 become a low level. According to this, in theclocked inverter 85 a on the left side, the PMOSFET 100 and the NMOSFET102 are turned on, the capacitors the setting operation of C6 and C7 iscarried out, and thereby the capacitors C6 and C7 are properly charged.Since the PMOSFET 100 and the NMOSFET 102 are off in the clockedinverter 85 b on the right side, the setting operation is not carriedout in the clocked inverter 85 b. In a second phase, a potential of thefirst control signal line 104 becomes a low level and the MOSFETs 100and 102 are turned off, therefore, the setting operation in the clockedinverter 85 a is completed.

Subsequently in the second setting operation, in the first phase, thepotentials of the second control signal line 105 and the clock signalline 96 become a high level while the potential of the inverted clocksignal line 95 becomes a low level. According to this, the PMOSFET 100and the NMOSFET 102 in the clocked inverter 85 b on the right side areturned on and the setting operation of the capacitors C6 and C7 iscarried out. In the second phase, the potential of the second controlsignal line 105 becomes a low level, and the setting operation in theclocked inverter 85 b is completed. In a normal operation, thepotentials of the first and the second control signal lines 104 and 105are maintained at a low level and the electric charges accumulated inthe capacitors C6 and C7 in each of the clocked inverters 85 a and 85 bare held, and in this state, a clock signal is supplied to the clocksignal and the inverted clock signal lines 96 and 95.

FIG. 20 is a circuit diagram showing another embodiment of the modifiedshift register 81 that includes the clocked inverters 85 a and 85 bshown in FIG. 18. In this drawing, the same portions as FIG. 18 aredenoted by the same reference numerals. The embodiment shown in FIG. 20is different from that shown in FIG. 18 in that a second initializationsignal line 108 a is provided in addition to the initialization signalline 108 (referred to as a first initialization signal line), and thegates of the MOSFETs 106 and 107 for initialization in the clockedinverter 85 b on the right side are connected to the secondinitialization signal line 108 a so that the initialization operationsin the clocked inverter 85 a on the left side and the clocked inverter85 b on the right side can be carried out separately.

FIG. 21 is a timing chart showing preferable changes in signals(potentials) of the respective portions in an initialization, a settingoperation and a normal operation in the embodiment shown in FIG. 20. Asshown in FIG. 21, in this embodiment, a first initialization operationis carried out before a first setting operation in which electriccharges are accumulated in the capacitors C6 and C7 in the clockedinverter 85 a on the left side of FIG. 20, and a second initializationoperation is carried out before a second setting operation in whichelectric charges are accumulated in the capacitors C6 and C7 in theclocked inverter 85 b on the right side.

In the first initialization operation, a potential of the firstinitialization signal line 108 becomes a high level, a potential of theclock signal line 96 being a low level, a potential of the invertedclock signal line 95 being a high level, and potentials of the firstcontrol signal line 104 and the second control signal line 105 being alow level. According to this, the NMOSFET 106 and the PMOSFET 107 in theclocked inverter 85 a are turned on, and the capacitors C6 and C7 in thecorrecting circuits 97 and 98 are initialized. The first settingoperation was described with reference to FIG. 19, therefore, theexplanation is omitted here.

In the second initialization operation, a potential of the secondinitialization signal line 108 a becomes a high level, the potential ofthe clock signal line 96 being a high level, the potential of theinverted clock signal line 95 being a low level, and the potentials ofthe first control signal line 104 and the second control signal line 105being a low level. According to this, the NMOSFET 106 and the PMOSFET107 in the clocked inverter 85 b are turned on, and the capacitors C6and C7 in the correcting circuits 97 and 98 are initialized. The secondsetting operation was described with reference to FIG. 19, therefore,the explanation is omitted here.

In the aforementioned embodiment, the initialization operation isdivided into the first initialization operation and the secondinitialization operation. Accordingly, the potentials of the clocksignal line 96 and the inverted clock signal line 95 can be controlledproperly in each initialization operation so that a high level potentialV_(CH) is inputted to the capacitor C6 connected to the gate of thePMOSFET 93 while a low level potential V_(CL) is inputted to thecapacitor C7 connected to the gate of the NMOSFET 94.

FIG. 22 is a circuit diagram showing another embodiment of the clockedinverter 85 a (85 b) shown in FIG. 18. In this drawing, the sameportions as FIG. 18 are denoted by the same reference numerals, and areexplained in no more details. In a clocked inverter 85 c, a terminal ofthe capacitor C6 on the opposite side to a terminal that is connected tothe gate of the PMOSFET 93 is connected through a PMOSFET 110 to theinverted clock signal line 95 while connected through a PMOSFET 111 to apotential V_(H)′ that is substantially equal to a high level potentialV_(CH) of an inverted clock signal. Similarly, a terminal of thecapacitor C7 on the opposite side to a terminal that is connected to thegate of the NMOSFET 94 is connected through an NMOSFET 112 to the clocksignal line 96 while connected through an NMOSFET 113 to a potentialV_(L)′ that is substantially equal to a low level potential V_(CL) of aclock signal. Gates of the MOSFETs 100, 111 and 112 are connectedthrough an inverter 114 to a control signal line 115, gates of theMOSFETs 102, 110 and 113 are connected directly to the control signalline 115. According to this, when a potential of the control signal line115 becomes high, the MOSFETs 100, 111, 102, and 113 are turned on whilethe MOSFETs 110 and 112 are turned off, and thereby electric charges areaccumulated in the capacitors C6 and C7 (setting operation). On theother hand, in the case of the potential of the control signal line 115being a low level, the MOSFETs 100, 111, 102, and 113 are turned offwhile the MOSFETs 110 and 112 are turned on, and thereby an invertedclock signal and a clock signal are supplied through the chargedcapacitors C6 and C7 to the gates of the PMOSFET 93 and the NMOSFET 94.Such an embodiment shown in FIG. 22 can be considered to be an examplein which the switches SW12, SW13, and SW16 to SW19 in the clockedinverter circuit 60 b shown in FIG. 16 are formed of the MOSFETs 100,102, and 110 to 113. It is needless to say that although the MOSFETs 106and 107 for initialization of the capacitors C6 and C7 shown in FIG. 18are not provided in this embodiment, they may be provided as required.

FIG. 23 is a circuit diagram showing a typical unit circuit in the firstlatch circuit 82 shown in FIG. 17. A unit circuit 120 comprises twoinverters 121 and 122 and two clocked inverters 123 and 124, and has afunction to latch a digitalized video signal in accordance with aselective signal from the shift register 81. The invention may beapplied to the clocked inverter 123 to which the video signal issupplied as an input signal, in the case of a high level potential ofthe video signal being lower than a high level power supply potentialVDD and/or a low level potential of the video signal being higher than alow level power supply potential VSS.

FIG. 24 is a circuit diagram showing an embodiment in which theinvention is applied to the clocked inverter 123 of the unit circuit 120shown in FIG. 23. The clocked inverter 85 c using a correcting circuitfor the MOSFET for clock signal synchronization is shown in FIG. 22.Meanwhile, in FIG. 24, a clocked inverter using a correcting circuit fora MOSFET to which an input signal is inputted is shown. The clockedinverter 123 comprises a PMOSFET 131 and an NMOSFET 132 whose drains areconnected to an output terminal OUT and in series with each other so asto constitute a CMOS inverter. Both gates of these MOSFETs 131 and 132are connected to an input terminal IN to which a video signal isinputted as an input signal. A source of the PMOSFET 131 is connectedthrough a PMOSFET 133 to a high level power supply potential VDD, asource of the NMOSFET 132 being connected through an NMOSFET 134 to alow level power supply potential VSS (VGND in this example). A selectivesignal from the shift register is inputted to gates of the PMOSFET 133and the NMOSFET 134. However, since an inverter 135 is provided to thegate of the PMOSFET 133, a signal with opposite polarity is inputted toeach of the MOSFETs 133 and 134.

Correcting circuits 136 and 137 are respectively connected between theinput terminal IN and the gates of the PMOSFET 131 and the NMOSFET 132.The correcting circuit 136 comprises a capacitor C8 connected betweenthe gate of the PMOSFET 131 and the input terminal IN, a diode-connectedPMOSFET 138 that has the substantially same threshold voltage as thePMOSFET 131, and a PMOSFET 139 that functions as a switch forselectively carrying out a setting operation. The PMOSFET 138 and thePMOSFET 139 are connected in series between a node N11 between thecapacitor C8 and the gate of the PMOSFET 131 and a high level powersupply potential VDD. Similarly, the correcting circuit 137 comprises acapacitor C9 connected between the gate of the NMOSFET 132 and the inputterminal IN, a diode-connected NMOSFET 140 that has the substantiallysame threshold voltage as the NMOSFET 132, and an NMOSFET 141 thatfunctions as a switch for selectively carrying out a setting operation.The NMOSFET 140 and the NMOSFET 141 are connected in series between anode N12 between the capacitor C9 and the gate of the NMOSFET 132 and alow level power supply potential VSS. In this embodiment, a gate of thePMOSFET 139 is connected to a P-channel control signal line 142, a gateof the NMOSFET 141 being connected to an N-channel control signal line143. However, in the case of the setting operation being carried out atthe same time in the PMOSFET and the NMOSFET as shown in FIG. 16 andFIG. 22, an inverter is provided to either the gate of the PMOSFET 139or the gate of the NMOSFET 141 similarly to the embodiment shown in FIG.18, and thereby only one control signal line can be used in common.

Furthermore, the node N11 between the capacitor C8 and the gate of thePMOSFET 131 is connected through an NMOSFET 144 to the low level powersupply potential VSS, the node N12 between the capacitor C9 and the gateof the NMOSFET 132 being connected through a PMOSFET 145 to the highlevel power supply potential VDD. The NMOSFET 144 is connected directlyto an initialization signal line 146, a gate of the PMOSFET 145 isconnected through an inverter 147 to the initialization signal line 146,and signals with opposite phases are inputted to the gates of theseMOSFETs 144 and 145. It is to be noted that the initialization signalline may be arranged separately as the embodiment shown in FIG. 12.

FIG. 25 is a timing chart showing preferable changes in signals(potentials) of the respective portions in an initialization, a settingoperation and a normal operation of thus constituted clocked inverter123 in the latch circuit. As shown in FIG. 25, an initializationoperation, an N-channel setting operation (setting operation of thecapacitor C9), a P-channel setting operation (setting operation of thecapacitor C8), and a normal operation are carried out in this order.Each of the N-channel setting operation and the P-channel settingoperation is composed of two phases. It is needless to say that theorder of the N-channel setting operation and the P-channel settingoperation can be changed over.

In the initialization operation, an initialization signal (146) becomeshigh, an input signal (video signal), a selective signal, and anN-channel control signal (143) being low and a P-channel control signal(142) being high. Since the P-channel control signal is a high level andthe N-channel control signal is a low level, the PMOSFET 139 and theNMOSFET 141 are off. When the initialization signal becomes high, theMOSFETs 144 and 145 are turned on and the capacitors C8 and C9 areinitialized (that is, a potential of the node N11 is lowered to a lowlevel power supply potential VSS whereas a potential of the node N12 isincreased to a high level power supply potential VDD). When theinitialization signal becomes a low level, the initialization operationis completed.

In the N-channel setting operation for accumulating electric charges inthe capacitor C9 connected to the gate of the NMOSFET 132, the N-channelcontrol signal (143) becomes high in a first phase while the videosignal (IN) remains at a low level. As a result, the NMOSFET 141 isturned on and a current flows from the input terminal IN to the lowlevel power supply potential VSS, and thereby the capacitor C9 ischarged. The N-channel control signal is maintained at a high level fora sufficient time for a voltage across the capacitor C9 to reach aproper value and for the NMOSFET 141 to turn off. In a second phase, theN-channel control signal becomes low and the N-channel setting operationis completed.

In the P-channel setting operation for accumulating electric charges inthe capacitor C8 connected to the gate of the PMOSFET 131, the videosignal (IN) becomes high in a first phase while the P-channel controlsignal (142) remains at a low level. As a result, the PMOSFET 139 isturned on and a current flows from the high level power supply potentialVDD to the input terminal IN, and thereby the capacitor C8 is charged.The P-channel control signal is maintained at a low level for asufficient time for a voltage across the capacitor C8 to reach a propervalue and for the PMOSFET 139 to turn off, and is then turned high in asecond phase. When the video signal becomes low, the normal operationcan be started. As shown in FIG. 25, in the normal operation, the videosignal and the selective signal are inputted, the P-channel controlsignal being high and the N-channel control signal being low. As setforth above, there are two types of circuits: the one in which acapacitor is connected directly to an input terminal IN as shown in FIG.5 and FIG. 7 and the other in which a capacitor is connected through aswitch to an input terminal IN as shown in FIG. 13 and FIG. 16. Bycombining these two types of circuits, various circuits can beconfigured and timing of the setting operation can be changedarbitrarily depending on the configuration of each circuit.

In the various embodiments of the invention described above, after asetting operation of a capacitor in a correcting circuit, a switchconnected between the capacitor and a power supply potential (VDD orVSS) is turned off, therefore, electric charges accumulated in thecapacitor are held. However, since some leakage current occurs actually,the setting operation is preferably carried out at a proper period. Forexample, in the case of the invention being applied to a transistor in ashift register of an active matrix circuit of a liquid crystal display,the setting operation may be carried out in a return period of aninputted video signal in which the shift register is not operated (seeFIG. 26 a).

Further, known is a display adopting a time gray scale method in whichgray scale is obtained by varying a total period of light emission ofeach pixel in one frame by selectively combining a plurality ofdifferent light emitting periods E1, E2, . . . in one frame period (inthe case of 4-bit display, for example, 16 gray scales can be achievedby combining E1 to E4, when it is supposed that E1 is the shortest lightemitting period and E2=2×E1, E3=4×E1, and E4=8×E1 are obtained). In adisplay adopting such a time gray scale method, for example, after dataindicating whether or not light is emitted in the light emitting periodE3 is written to a memory for each pixel, there is a period in which adriver circuit is not operated such as a period before or after writingof data whether or not light is emitted in the light emitting period E4(see FIG. 26 b). The aforementioned setting operation of the correctingcircuit can be carried out in such a period in which the driver circuitis not operated. It is to be noted that the setting operation is notnecessarily carried out in all the correcting circuits at a time, and itmay be carried out at different timing in each correcting circuit. Inaddition, a signal is shifted and transferred in sequence in the shiftregister shown in FIG. 17 or FIG. 18. Therefore, the setting operationof a correcting circuit may be carried out by using a signal in theseveral stages before.

The invention can be applied to a logical circuit such as a NANDcircuit, a NOR circuit, and a transfer gate. FIG. 27 is a circuitdiagram showing an embodiment in which the invention is applied to atransistor constituting a NAND circuit. FIG. 28 is a circuit diagramshowing an embodiment in which the invention is applied to a transistorconstituting a NOR circuit.

A digital circuit 150 shown in FIG. 27 comprises two PMOSFETs 151 and152 that are connected in parallel, and two NMOSFETs 153 and 154 thatare connected in series, and these four MOSFETs 151 to 154 constitute aNAND circuit. More specifically, gates of the PMOSFET 151 and theNMOSFET 153 are connected to a first input terminal IN1, gates of thePMOSFET 152 and the NMOSFET 154 being connected to a second inputterminal IN2. Sources of the PMOSFETs 151 and 152 are both connected toa high level power supply potential VDD, both drains thereof beingconnected to a drain of the NMOSFET 154 as well as an output terminalOUT. A source of the NMOSFET 154 is connected to a drain of the NMOSFET153, a source of the NMOSFET 153 being connected to a low level powersupply potential VSS (V_(GND) in this example). Such a NAND circuit isknown well in this field.

According to the invention, correcting circuits 155 to 158 are providedfor the MOSFETs 151 to 154 respectively. Similarly to the aforementionedembodiments, each of the correcting circuits 155 to 158 comprises acapacitor connected to the gate of the corresponding MOSFET, adiode-connected MOSFET that has the same polarity and the substantiallysame threshold voltage as the corresponding MOSFET, and a switch that isconnected in series with the diode-connected MOSFET. Operations andeffects of such correcting circuits 155 to 158 are similar to those ofthe embodiments described above, therefore, the explanation thereof isomitted here.

A digital circuit 160 shown in FIG. 28 comprises two PMOSFETs 161 and162 that are connected in series, and two NMOSFETs 163 and 164 that areconnected in parallel, and these four MOSFETs 161 to 164 constitute aNOR circuit. More specifically, gates of the PMOSFET 161 and the NMOSFET163 are connected to a first input terminal IN1, gates of the PMOSFET162 and the NMOSFET 164 being connected to a second input terminal IN2.A source of the PMOSFET 161 is connected to a high level power supplypotential VDD, a drain thereof being connected to a source of thePMOSFET 162. A drain of the PMOSFET 162 is connected to drains of theNMOSFETs 163 and 164 as well as an output terminal OUT. Sources of theNMOSFETs 163 and 164 are both connected to a low level power supplypotential VSS (V_(GND) in this example). Such a NOR circuit is knownwell in this field.

According to the invention, correcting circuits 165 to 168 are providedfor the MOSFETs 161 to 164 respectively. Similarly to the aforementionedembodiments, each of the correcting circuits 165 to 168 comprises acapacitor connected to the gate of the corresponding MOSFET, adiode-connected MOSFET that has the same polarity and the substantiallysame threshold voltage as the corresponding MOSFET, and a switch that isconnected in series with the diode-connected MOSFET. Operations andeffects of such correcting circuits 165 to 168 are similar to those ofthe embodiments described above, therefore, the explanation thereof isomitted here.

Described above is a preferable embodiment of a digital circuit having aswitching circuit using a transistor, which is capable of turning on/offthe transistor assuredly even in the case of an amplitude of an inputsignal being smaller than a power supply voltage (difference between ahigh level power supply potential and a low level power supplypotential). When the setting operation is changed properly, theaforementioned embodiment can respond to the case of operation speed ofthe transistor being preferably improved when the power supply voltageis not sufficiently large with respect to the absolute value of athreshold voltage of the transistor. FIG. 29 shows another embodiment ofa digital circuit that can carry out such a setting operation. In thisembodiment, the same portions as the embodiment shown in FIG. 5 aredenoted by the same reference numerals, and are described in no moredetails.

In a digital circuit (inverter circuit) 30 e shown in FIG. 29, the nodeN5 between the gate of the PMOSFET 32 and the capacitor C2 is connectedthrough a switch SW20 to a low level potential V_(L)″, the node N6between the gate of the NMOSFET 33 and the capacitor C3 being connectedthrough a switch SW21 to a high level potential V_(H)″. The low levelpotential V_(L)″ can be made equal to a low level power supply potentialVSS and the high level potential V_(H)″ can be made equal to, forinstance, a high level power supply potential VDD. In that case, thedigital circuit 30 e becomes the same as the digital circuit 30 c shownin FIG. 10.

A setting and normal operations of thus constituted digital circuit 30 ewill be explained below. It is herein assumed that a low level inputpotential V_(INL) is equal to the low level power supply potential VSS(V_(GND) in this example) and a high level input potential V_(INH) isequal to the high level power supply potential VDD.

As shown in FIG. 30 a, in a first setting operation for the capacitorC2, the switches SW2, SW3 and SW21 are turned off, and in this state,the switch SW20 is turned on and a high level input potential V_(INH) isinputted to the input terminal IN. Then, a current flows in thedirection shown by an arrow in the drawing, and thereby the capacitor C2is charged so that the input terminal IN side becomes high and the gateside of the PMOSFET 32 becomes low. Subsequently, as shown in FIG. 30 b,in a second setting operation, the switch SW20 is turned off and theswitch SW2 is turned on while the high level input potential V_(INH)being inputted to the input terminal IN. Accordingly, the capacitor C2is discharged and a current flows in the direction shown by an arrow inthe drawing. When a voltage across the capacitor C2 becomes equal to athreshold voltage V_(THP) of the PMOSFET 35, the current is stopped. Itis to be noted that the switch SW2 may be turned on in the first settingoperation. The low level potential V_(L)″ is not necessarily equal tothe VSS and has only to be a value that allows the capacitor C2 to becharged, in the first setting operation, at a voltage larger than thethreshold voltage V_(THP) of the PMOSFET 35 (namely, of the PMOSFET 32).The first setting operation can be considered as an initializationoperation.

Similarly, as shown in FIG. 31 a, in a first setting operation for thecapacitor C3, the switches SW2, SW3 and SW20 are turned off, and in thisstate, the switch SW21 is turned on and a low level input potentialV_(INL) is inputted to the input terminal IN. Accordingly, a currentflows in the direction shown by an arrow in the drawing, and thereby thecapacitor C3 is charged so that the input terminal IN side becomes lowand the gate side of the NMOSFET 33 becomes high. Subsequently, in asecond setting operation, the switch SW2 1 is turned off and the switchSW3 is turned on while the low level input potential V_(INL) beinginputted to the input terminal IN. Accordingly, the capacitor C3 isdischarged and a current flows in the direction shown by an arrow inFIG. 31 b. When a voltage across the capacitor C3 becomes equal to athreshold voltage V_(THN) of the NMOSFET 37, the current is stopped. Itis to be noted that the switch SW3 may be turned on in the first settingoperation. The high level potential V_(H)″ is not necessarily equal tothe VDD and has only to be a value that allows the capacitor C3 to becharged, in the first setting operation, at a voltage larger than thethreshold voltage V_(THN) of the NMOSFET 37 (namely, of the NMOSFET 33).

After the capacitors C2 and C3 are thus charged, in a normal operation,the switches SW2, SW3, SW20, and SW21 are turned off, and an inputsignal that oscillates between a high level input potential V_(INH) anda low level input potential V_(INL) is inputted to the input terminalIN. In the case of a high level input potential V_(INH) being inputted,as shown in FIG. 32 a, a gate potential of the PMOSFET 32V_(INH)−|V_(THP)| becomes equal to VDD−|V_(THP)|, and a gate-sourcevoltage V_(GS) thereof becomes equal to −|V_(THP)|, and thereby thePMOSFET 32 is turned off. On the other hand, a gate potential of theNMOSFET 33 V_(INH)+|V_(THN)| becomes equal to VDD+|V_(THN)|. Therefore,a voltage obtained by subtracting the V_(THN) from a gate-source voltageV_(GS) of the NMOSFET 33 becomes equal to the VDD, and thereby a voltagethat is large enough to flow a sufficient current to the NMOSFET 33 toturn it on at high-speed is assured.

Similarly, in the case of a low level input potential V_(INL) beinginputted to the input terminal IN, as shown in FIG. 32 b, a gatepotential of the NMOSFET 33 V_(INL)+|V_(THN)| becomes equal toV_(GND)+|V_(THN)|, and a gate-source voltage V_(GS) thereof becomesequal to |V_(THN)|, and thereby the NMOSFET 33 is turned off. On theother hand, a gate potential of the PMOSFET 32 V_(INL)−|V_(THP)| becomesequal to V_(GND)−|V_(THP)|. Therefore, a voltage obtained by subtractingthe V_(THP) from a gate-source voltage V_(GS) of the PMOSFET 32 becomesequal to −VDD, and thereby a voltage (absolute value) that is largeenough to flow a sufficient current to the PMOSFET 32 to turn on withhigh-speed is assured.

As set forth above, in the embodiments described with reference to FIGS.29 to 32, the capacitors C2 and C3 in the correcting circuit can becharged so as to correct a DC level of an input signal in order toimprove on-operation speed of the corresponding MOSFETs 32 and 33.Accordingly, a power supply voltage can be lowered without loweringcircuit operation speed, leading to reduction in power consumption.Although a low level input potential V_(INL) is equal to a low levelpower supply potential VSS (V_(GND) in this example) while a high levelinput potential V_(INH) is equal to a high level power supply potentialVDD in the above description, the invention is not limited to this. Inthe above circuit, in general, the absolute value of a voltage of thecapacitor C2 becomes |V_(THP)|−(VDD−V_(INH)) after a setting operationwhereas the absolute value of a voltage of the capacitor C3 becomes|V_(THN)|−(V_(INL)−VSS) after a setting operation. In an off-state,V_(GS)=threshold voltage is obtained in both the PMOSFET 32 and theNMOSFET 33, and both the PMOSFET 32 and the NMOSFET 33 are barely turnedoff. However, in an on-state, |V_(GS)|=|thresholdvoltage|+V_(INH)−V_(INL) is obtained.

In the digital circuit 30 e shown in FIG. 29, setting operations of thecapacitor C2 connected to the gate of the PMOSFET 32 and the capacitorC3 connected to the gate of the NMOSFET 33 are carried out separately byvarying an input signal potential inputted to the input terminal IN.However, it is preferable that these setting operations can be carriedout at the same time. Such a digital circuit is shown in FIG. 33. Notethat this embodiment with reference to FIG. 33 is an embodiment to whichthe digital circuit 30 d shown in FIG. 13 is applied, and in FIG. 33,the same portions as FIG. 13 and FIG. 29 are denoted by the samereference numerals and described in no more details.

In a digital circuit 30 f shown in FIG. 33, a terminal of the capacitorC2 on the opposite side to a terminal that is connected to the gate ofthe PMOSFET 32 is connected through the switch SW8 to the input terminalIN while connected through the switch SW9 to a high level power supplypotential VDD. Similarly, a terminal of the capacitor C3 on the oppositeside to a terminal that is connected to the gate of the NMOSFET 33 isconnected through the switch SW10 to the input terminal IN whileconnected through the switch SW11 to a low level power supply potentialVSS.

A setting and normal operations of thus constituted digital circuit 30 fwill be explained below. It is herein assumed that, similarly to thedescription of the operation of the digital circuit 30 e, a low levelinput potential V_(INL) is equal to the low level power supply potentialVSS (V_(GND) in this example) and a high level input potential V_(INH)is equal to the high level power supply potential VDD.

As shown in FIG. 34 a, in a first setting operation, the switches SW2,SW3, SW8, and SW10 are turned off, and the switches SW9, SW11, SW20, andSW21 are turned on. Then, currents flow in the directions shown byarrows in the drawing, and thereby the capacitor C2 is charged so thatthe input terminal IN side becomes high and the gate side of the PMOSFET32 becomes low, while the capacitor C3 is charged so that the inputterminal IN side becomes low and the gate side of the NMOSFET 33 becomeshigh. The first setting operation can be considered as an initializationoperation.

As shown in FIG. 34 b, in a second setting operation, the switches SW2,SW3, SW9, and SW11 are turned on, and the switches SW8, SW10, SW20, andSW21 are turned off. According to this, the capacitors C2 and C3 aredischarged and currents flow in the directions shown by arrows in thedrawing. The respective currents stop when a voltage across thecapacitor C2 becomes equal to a threshold voltage of the PMOSFET 35 anda voltage across the capacitor C3 becomes equal to a threshold voltageof the NMOSFET 37.

After the setting of the capacitors C2 and C3, in a normal operation,the switches SW2, SW3, SW9, SW11, SW20, and SW21 are turned off whilethe switches SW8 and SW10 are turned on, and an input signal is inputtedto the input terminal IN as shown in FIG. 35. The operation of theMOSFETs 32 and 33 in this case is the same as that described in FIG. 32a and FIG. 32 b, therefore, the explanation is omitted herein. In thisembodiment, a low level input potential V_(INL) is equal to a low levelpower supply potential VSS and a high level input potential V_(INH) isequal to a high level power supply potential VDD, and thus thecapacitors C2 and C3 are connected through the switches SW9 and SW11 tothe high level power supply potential VDD and the low level power supplypotential VSS, respectively. However, if this is not the case, thecapacitors C2 and C3 may be connected through the switches SW9 and SW11to a potential that is substantially equal to the high level inputpotential V_(INH) and a potential that is substantially equal to the lowlevel input potential V_(INL), respectively.

Although the invention has been fully described with reference to theembodiments, the invention is not limited to the embodiments that areshown by way of example. It is needless to say that various changes andmodifications will be apparent to those skilled in the art unless suchchanges and modifications depart from the scope of the invention definedin claims.

For instance, although a low level power supply potential VSS is aground potential V_(GND) and a high level power supply potential VDD isa potential higher than the V_(GND) in the above embodiments, otherpotentials can be adopted such that a high level power supply potentialVDD is a ground potential V_(GND) and a low level power supply potentialVSS is a potential lower than the ground potential V_(GND). Furthermore,although the MOSFET is used as a transistor in the above embodiments,other transistors such as a bipolar transistor and other types of FETscan also be employed. A transistor may adopt any configuration,material, and manufacturing method, for example, may use a normal singlecrystalline substrate or an SOI (silicon on insulator) substrate.Moreover, a thin film transistor (TFT) using amorphous silicon,polysilicon and the like may be employed as well as a transistor usingan organic semiconductor or a carbon nanotube. In addition, thetransistor may be formed on a glass substrate, a quartz substrate, aplastic substrate or other substrates.

INDUSTRIAL APPLICABILITY

As set forth above, the digital circuit based on the invention comprisesa switching circuit having a first transistor such as a MOSFET suppliedwith a power supply potential, and a correcting circuit connectedbetween an input terminal inputted with an input signal and a controlterminal (gate) of the first transistor. The correcting circuit has a) acapacitor connected between the control terminal of the first transistorand the input terminal and b) at least one switch for determining aconduction path for setting, in a setting operation prior to a normaloperation, electric charges that are accumulated in the capacitor sothat a voltage across thereof may be a predetermined value. In a normaloperation, a state of the at least one switch is set so as to hold thevoltage across the capacitor. Accordingly, without the correctingcircuit, the switching circuit does not operate normally due to adifference between an input potential level and a power supply potentiallevel (for instance, a high level input potential is lower than a highlevel power supply potential), or in the case of the transistor beingnot operated with high-speed owing to the power supply voltage being notsufficiently large with respect to a threshold voltage of the transistor(for instance, the power supply voltage of 3.3 V and the thresholdvoltage of the transistor of 3 V), when the voltage across the capacitoris properly set in the setting operation and the set voltage (orpotential) is held in the normal operation, a DC level of the inputsignal can be corrected properly and thereby a preferable circuitoperation can be realized. Since electric charges of the capacitor areheld in the normal operation, there is no concern of the capacitoradversely affecting on the dynamic characteristics of the digitalcircuit (that is, operation speed is not lowered). On the contrary, thecapacitor, being connected in series with parasitic capacitance of thetransistor to lower total capacitance, can contribute to improve thedynamic characteristics. Furthermore, since there is no need tofrequently carry out the setting operation, power consumption due to thesetting operation is only slight. Preferably, in order that the voltageof the capacitor can reflect the threshold voltage of the correspondingtransistor, the correcting circuit further includes a diode-connectedsecond transistor that is provided between a node between the capacitorand the control terminal of the first transistor and the power supplypotential, and has the substantially same threshold voltage as the firsttransistor, and a switch that is connected in series with thediode-connected second transistor.

The invention can be applied to electronic apparatuses such as adesktop, floor standing, or wall hanging display, a video camera, adigital camera, a goggle type display (head mounted display), anavigation system, an audio reproducing device (an in-car audio system,an audio component set, and the like), a laptop personal computer, agame player, a portable information terminal (a mobile computer, amobile phone, a portable game player, an electronic book, and the like),and an image reproducing device provided with a recording medium(specifically, a device that reproduces an image or a still imagerecorded in a recording medium such as a Digital Versatile Disc (DVD)and includes a display capable of displaying the reproduced images).Specific examples of these electronic apparatuses are shown in FIG. 38 ato FIG. 38 h.

FIG. 38 a shows a desktop, floor standing, or wall hanging display thatincludes a housing 13001, a supporting base 13002, a display portion13003, a speaker portion 13004, a video input terminal 13005, and thelike. The invention can be applied to an electric circuit thatconstitutes the display portion 13003. Such a display can be used as aninformation display device for personal computer, TV broadcastreception, advertising display and the like.

FIG. 38 b shows a digital still camera that includes a main body 13101,a display portion 13102, an image receiving portion 13103, operatingkeys 13104, an external connecting port 13105, a shutter 13106, and thelike. The invention can be applied to an electric circuit thatconstitutes the display portion 13102.

FIG. 38 c shows a laptop personal computer that includes a main body13201, a housing 13202, a display portion 13203, a keyboard 13204, anexternal connecting port 13205, a pointing mouse 13206, and the like.The invention can be applied to an electric circuit that constitutes thedisplay portion 13203.

FIG. 38 d shows a mobile computer that includes a main body 13301, adisplay portion 13302, a switch 13303, operating keys 13304, an infraredport 13305, and the like. The invention can be applied to an electriccircuit that constitutes the display portion 13302.

FIG. 38 e shows a portable image reproducing device provided with arecording medium (specifically a DVD reproducing device), that includesa main body 13401, a housing 13402, a first display portion 13403, asecond display portion 13404, a recording medium (such as a DVD) readingportion 13405, an operating key 13406, a speaker portion 13407, and thelike. The first display portion 13403 displays mainly image data whereasthe second display portion 13404 displays mainly character data. Theinvention can be applied to an electric circuit that constitutes thefirst and the second display portions 13403 and 13404. It is to be notedthat the image reproducing device provided with a recording mediumincludes a home game player and the like.

FIG. 38 f shows a goggle type display (head mounted display) thatincludes a main body 13501, a display portion 13502, and an arm portion13503. The invention can be applied to an electric circuit thatconstitutes the display portion 13502.

FIG. 38 g shows a video camera that includes a main body 13601, adisplay portion 13602, a housing 13603, an external connecting port13604, a remote control receiving portion 13605, an image receivingportion 13606, a battery 13607, an audio input portion 13608, operatingkeys 13609, and the like. The invention can be applied to an electriccircuit that constitutes the display portion 13602.

FIG. 38 h shows a mobile phone that includes a main body 13701, ahousing 13702, a display portion 13703, an audio input portion 13704, anaudio output portion 13705, an operating key 13706, an externalconnecting port 13707, an antenna 13708, and the like. The invention canbe applied to an electric circuit that constitutes the display portion13703.

A display portion of the aforementioned electronic apparatuses may be aself-light emitting type using in each pixel a light emitting elementsuch as an LED or an organic EL, or may be formed, such as a liquidcrystal display, by using another light source such as a backlight. Inthe case of the self-light emitting type, the display portion can bemade thinner than that of the liquid crystal display without requiring abacklight.

The aforementioned electronic apparatuses are becoming to be more usedfor displaying data distributed through a telecommunication path such asInternet and a CATV (Cable Television System), and in particular usedfor displaying moving pictures. The self-light emitting display portionis suitable for displaying moving pictures since the light emittingmaterial such as an organic EL can exhibit a remarkably high response.When the luminance of the light emitting material is improved in thefuture, it can be used for a front type or rear type projector bymagnifying and projecting outputted light including image data by a lensand the like.

Since light emitting parts consume power in a self-light emittingdisplay portion, data is desirably displayed so that the light emittingparts occupy as small area as possible. Accordingly, in the case where aself-light emitting type is adopted for a display portion that mainlydisplays character data, such as the one of a mobile phone or an audioreproducing device, it is preferably operated so that the character dataemits light by using non-light emitting parts as background.

As set forth above, the application range of the invention is so widethat it can be applied to electronic apparatuses of all fields.

1. A digital circuit comprising: a switching circuit electricallyconnected between an input terminal and an output terminal; and acorrecting circuit electrically connected between the input terminal andthe switching circuit, wherein: the switching circuit comprises a firsttransistor that includes a first terminal, a second terminal, and acontrol terminal a first power supply potential is inputted to the firstterminal of the first transistor, and a signal at the output terminaldepends on whether the first transistor is ON or OFF; an input signalthat oscillates between a first input potential for turning off thefirst transistor and a second input potential for turning on the firsttransistor is inputted to the input terminal; the correcting circuitcomprises a capacitor, a second transistor and at least one switch, withone terminal of the capacitor being electrically connected to the inputterminal and the other terminal of the capacitor being electricallyconnected to the control terminal of the first transistor and a secondterminal of the second transistor; the second transistor includes afirst terminal, the second terminal, and a control terminal, and thesecond terminal of the second transistor and the control terminal of thesecond transistor are connected to each other; when the switch is ON,the switch is provided for controlling electric charges that areaccumulated in the capacitor so that a voltage across the capacitorbecomes predetermined value, and when the switch is OFF, the switch isprovided to hold a voltage across the capacitor.
 2. The digital circuitaccording to claim 1, the second transistor has the same conductivityand the substantially same threshold voltage as the first transistor;the first power supply potential is inputted to the first terminal ofthe second transistor; and the second terminal of the second transistoris connected to a node between the capacitor and the control terminal ofthe first transistor.
 3. The digital circuit according to claim 2,wherein a potential substantially equal to the first input potential isinputted to the one terminal of the capacitor while the switch is onuntil the second transistor is turned off.
 4. The digital circuitaccording to claim 2, wherein a rectifier element is connected inparallel with the second transistor such that the forward directionthereof is opposite to the forward direction of the second transistor.5. The digital circuit according to claim 4, wherein the rectifierelement comprises a diode-connected transistor that has the sameconductivity as the second transistor.
 6. The digital circuit accordingto claim 1, wherein the one terminal of the capacitor is connectedthrough a second switch to the input terminal.
 7. The digital circuitaccording to claim 2, wherein the node between the capacitor and thecontrol terminal of the first transistor is connected through a secondswitch to a predetermined potential.
 8. The digital circuit according toclaim 7, wherein the predetermined potential is a second power supplypotential that is different from the first power supply potential. 9.The digital circuit according to claim 8, wherein the first inputpotential is equal to the first power supply potential.
 10. The digitalcircuit according to claim 1, wherein the switching circuit is aninverter circuit.
 11. The digital circuit according to claim 1, whereinthe switching circuit is a CMOS inverter circuit and the firsttransistor is a MOSFET.
 12. The digital circuit according to claim 1,wherein the switching circuit is a clocked inverter circuit.
 13. Thedigital circuit according to claim 12, wherein the first transistor is atransistor for clock signal synchronization that is connected in serieswith a transistor included in an inverter.
 14. The digital circuitaccording to claim 1, wherein the at least one switch comprises asemiconductor element.
 15. The digital circuit according to claim 1,wherein a second switch is connected in parallel with the capacitor. 16.The digital circuit according to claim 1, wherein the switching circuitincludes another transistor which has a different polarity from that ofthe first transistor.
 17. An electronic apparatus including the digitalcircuit according to claim
 1. 18. The electronic apparatus according toclaim 17, wherein the electronic apparatus includes a display portionand the digital circuit is used in the display portion.
 19. Theelectronic apparatus according to claim 18, wherein the electronicapparatus is selected from a desktop, floor standing or wall hangingdisplay, a video camera, a digital camera, a goggle type display, anavigation system, an audio reproducing device, a laptop personalcomputer, a game player, a portable information terminal, and an imagereproducing device provided with a recording medium.
 20. A digitalcircuit comprising: a switching circuit electrically connected betweenan input terminal and an output terminal; and a correcting circuitelectrically connected between the input terminal and the switchingcircuit, wherein: the switching circuit comprises a first transistorthat includes a first terminal, a second terminal, and a controlterminal; a first power supply potential is inputted to the firstterminal of the first transistor, and a signal at the output terminaldepends on whether the first transistor is ON or OFF; an input signalthat oscillates between a first input potential for turning off thefirst transistor and a second input potential for turning on the firsttransistor is inputted to the input terminal; the correcting circuitcomprises a capacitor, a second transistor and a first switch, with oneterminal of the capacitor being electrically connected to the inputterminal and the other terminal of the capacitor being electricallyconnected to the control terminal of the first transistor and a secondterminal of the second transistor; the second transistor has the sameconductivity and the substantially same threshold voltage as the firsttransistor; and the second transistor includes a first terminal, thesecond terminal, and a control terminal, and the second terminal of thesecond transistor and the control terminal of the second transistor areconnected to each other.
 21. The digital circuit according to claim 20,wherein a rectifier element is connected in parallel with the secondtransistor such that the forward direction thereof is opposite to theforward direction of the second transistor.
 22. The digital circuitaccording to claim 21, wherein the rectifier element comprises adiode-connected transistor that has the same conductivity as the secondtransistor.
 23. The digital circuit according to claim 20, wherein theone terminal of the capacitor is connected through a second switch tothe input terminal.
 24. The digital circuit according to claim 20,wherein the switching circuit is an inverter circuit.
 25. The digitalcircuit according to claim 20, wherein the switching circuit is a CMOSinverter circuit and the first transistor is a MOSFET.
 26. The digitalcircuit according to claim 20, wherein the switching circuit is aclocked inverter circuit.
 27. The digital circuit according to claim 20,wherein the first switch comprises a semiconductor element.
 28. Thedigital circuit according to claim 20, wherein a second switch isconnected in parallel with the capacitor.
 29. An electronic apparatusincluding the digital circuit according to claim
 20. 30. A digitalcircuit comprising: a switching circuit electrically connected betweenan input terminal and an output terminal; and a correcting circuitelectrically connected between the input terminal and the switchingcircuit, wherein: the switching circuit comprises a first transistorthat includes a first terminal, a second terminal, and a controlterminal; a signal at the output terminal depends on whether the firsttransistor is ON or OFF; the correcting circuit comprises a capacitor, asecond transistor and at least one switch, with one terminal of thecapacitor being electrically connected to the input terminal and theother terminal of the capacitor being electrically connected to thecontrol terminal of the first transistor and a second terminal of thesecond transistor; the second transistor includes a first terminal, thesecond terminal, and a control terminal and the second terminal of thesecond transistor and the control terminal of the second transistor areconnected to each other; when the switch is ON, the switch is providedfor controlling electric charges that are accumulated in the capacitorso that a voltage across the capacitor becomes predetermined value, andwhen the switch is OFF, the switch is provided to hold a voltage acrossthe capacitor.
 31. A digital circuit comprising: a switching circuitelectrically connected between an input terminal and an output terminal;and a correcting circuit electrically connected between the inputterminal and the switching circuit, wherein: the switching circuitcomprises a first transistor that includes a first terminal, a secondterminal, and a control terminal; a signal at the output terminaldepends on whether the first transistor is ON or OFF; the correctingcircuit comprises a capacitor, a second transistor and a first switch,with one terminal of the capacitor being electrically connected to theinput terminal and the other terminal of the capacitor beingelectrically connected to the control terminal of the first transistorand a second terminal of the second transistor; the second transistorhas the same conductivity and the substantially same threshold voltageas the first transistor; and the second transistor includes a firstterminal, the second terminal, and a control terminal, and the secondterminal of the second transistor and the control terminal of the secondtransistor are connected to each other.
 32. The digital circuitaccording to claim 1, wherein the switch is electrically connected withthe first terminal of the second transistor.
 33. The digital circuitaccording to claim 20, wherein the first switch is electricallyconnected with the first terminal of the second transistor.
 34. Thedigital circuit according to claim 30, wherein the switch iselectrically connected with the first terminal of the second transistor.35. The digital circuit according to claim 31, wherein the first switchis electrically connected with the first terminal of the secondtransistor.
 36. The digital circuit according to claim 1, wherein theother terminal of the capacitor is electrically connected to the controlterminal of the first transistor directly.
 37. The digital circuitaccording to claim 20, wherein the other terminal of the capacitor iselectrically connected to the control terminal of the first transistordirectly.
 38. The digital circuit according to claim 30, wherein theother terminal of the capacitor is electrically connected to the controlterminal of the first transistor directly.
 39. The digital circuitaccording to claim 31, wherein the other terminal of the capacitor iselectrically connected to the control terminal of the first transistordirectly.